Granby Town

The Fourth Core

"Where the Amherst-Granby road passes over the range a single house (C. Harris's) stands on the west side. Eighty rods S. 65° E. of this house, on a wood road at the south foot of a high conglomerate bluff, is a small, nearly circular outcrop of trap with the adjoining sandstone exceptionally altered on the contact. The coaly plant remains which it contains are largely changed into graphite.

The Fifth Core

In the field west of the only other house on this road (A. Convere's) and about 30 rods N. 10° W. of the house is a ridge of the trap 8 rods wide, 25 rods long, running N. 50° E. a few rods north of the tuff bed.

The Sixth Core

In the same field with the last and about 35 rods S. 75° W. from it is another high ridge, 14 rods wide and 30 rods long, having about the same direction of its longer axis, the same relation to the tuff bed, and the same fine-grained, dark-gray diabase without amygdaloid as have the other occurrences. Like them all, it bakes the sandstone strongly and seems to be wholly surrounded by the sandstone. It is more jointed and weathered than usual. About 5 rods southeast is a dike of the diabase 10 feet wide and about 5 rods long, which runs north and south parallel with a fault plane a little farther east. This is very probably an offshoot from the latter throat, but the junction is covered.

The Seventh Core

About 200 rods north of J. McGrath's this plug occurs, at the eastern terminus of the band of diabase-tuff which crosses the Notch road and enters Granby from the west. This terminus is caused by the intersection of the tuff band by one of the main north-south faults which cross the trap ridge, and the small plug of trap came up just at this crossing. It is only 5 or (J rods in cross-section. The rock is black, fresh, compact, only the larger and earlier generation of feldspars showing as fine lines with a stron"- lens. The few small cavities show a white, compact, fibrous zeolite. Under the microscope the rock shows the usual structure, with. the following ])eeuliarities: The area surrounding the cavities is of much finer "-rain than the remainder. The large feldspars are almost always made up of only two broad individuals. An interstitial groundmass exists in considerable quantity and decomposes to a red-brown mass. The cavities contain (1) calcite, (2) diabantite, (3) wavy interstratifications of diabantite and natrolite, (4) natrolite, (5) a red decomposition product of diabantite. The whole slide is full of the finest feathery groups of magnetite octahedra.

The Eighth Oore

One will find this large and interesting mass by starting from the southern boundary of the tuff, just across the brook south of the first branching of the road after passing The Notch. The sandstone rises in a low bluff facing north, and may be followed due east 100 rods across the field and through the woods to a deep, dry gorge running north. Following this up across the tuff, one comes upon the trap at the head of the gorge and can follow the line of contact very closely round in a great circle to the point of starting, and it is plain in several places that the trap has come up through the sandstone and is not a bed in it, and that it has come up on the line between the sandstone and the tuff, intruding partly on the area of the one and partly on that of the other. No amygdaloid was observed, and the diabase is very fresh, dark-gray with shade of brown, fine-grained, the earlier generation of feldspars being just visible to the eye, and then striation discernible with a strong lens.

The induration of the tuff was very marked on its contact with the diabase on the south side of the block. This would be the upper surface of the bed if it had been a contemporaneous flow, and in this case there would have been no trace of baking of the adjacent tuff.

South Hadley

An inspection of the map will show the next, or Black Rock, core to consist of a great rounded mass which is situated south of the Holyoke House and sends off to the east a long tail-like dike. If this be followed east half a mile to a point beyond the Moody Corners wood road it will be seen to be bordered on the south by a rounded mass of diabase lithologically different from anything" found elsewhere in the valley, and very peculiar. It has along dike projecting- to the east, and is thus paddle-shaped and rudely resembles the much larger Black Rock core. It is an aphanitic diabase, which is full of grains of quartz, microcline, orthoclase, etc. - so full that in breaking hand specimens from every ledge across its width, 650 feet, not one was found that did not contain many grains. It is as if the lava had broken up through unconsolidated sandstones or flowed over beds of sand, taking up a great quantity of the latter in its progress ; and as the tuffs contain a ceiiain quantity of the same granitic sand, and as this had often been relied on as a means of distinguishing the tuff from weathered outcrops of the trap in the field, a new difficulty was added, and weathered outcrops of this trap were with great difficulty distinguished. Indeed, in my first section across this area I was in great perplexity, and the . peculiar pitchstonelike appearance of the rock, quite new in this region, added thereto. Still, I decided before cutting sections that the rock must be a lava and not a tuff.

Petkographical Description

The specimens described here were taken on a section through the middle of the dike, where a wood road goes north a short distance east of Moody Corners and crosses the brook where a branch enters it. From this point the first outcrop of the trap is visible, and going north across a cleared area and then a short distance through the woods to a second cleared field one finds abundant exposm-es of the trap, and to the north long ridges of the sandstone appear. The first section was taken from the north cleared field, 23 feet south of north border of the diabase. It is a clear, black, aphanitic rock of unusual freshness and slightly pitchy luster, and contains in great quantity inclusions of granitic quartz, and rarely orthoclase, of the same size as the grains in the adjacent sandstone (up to As many as three or four to a square centimeter occur, and the lens shows many more of smaller size. Fragments of a glassy triclinic feldspar, perfectly fresh, and the larger groups of plagioclase of earlier consolidation are also visible. The quartz is colorless or slightly blue, of strong greasy luster, without fissures and with rounded outline, or much fissured and then yellowish. A single grain, square, was made up mostly of a flesh-colored feldspar, but nearly a quarter of its mass was of the same greasy, bluish quartz.

The Newer Cokes And Shoet Dikes

The microscope reveals anorthite, labradorite, augite, fayalite, magnetite, liematite, and, as inclusions, quartz, orthoclase, microcline, albite, rutile, muscovite, and biotite, and fragments of granite and amphibolite. The texture of the rock is entirely unlike any other occurrence among the eruptives of the region. The original minerals enumerated appear as fresh as in a modern lava, distantly, often very distantly, scattered in a finely granular ground.

Anorthite. - The porphyritic feldspars of the first consolidation are often as much as 2°"" across. They seem to me to have shot out rapidly in thin plates, which are often much bent and broken and the parts moved away from each other, showing what is otherwise proved below, that they were formed in the liquid magma and moved some distance in it before its entire consolidation. They are of glassy clearness, and show not the faintest trace of alteration. They are bounded by perfectly smooth crystal faces, except where deep angular or rounded offshoots of the groundmass extend into them. Inclusions of this groundmass are very abundant and variously arranged - at times collected in the center, at times in concentric lines marking old surfaces of the crystal, which often differ from the final form; at times quite regularly arranged in I, T, and L shaped masses, conforming to the cleavage planes with great regularity, the masses being of quite uniform size. The crystals show lines of growth of great delicacy, which are curiously distm-bed as they bend in to surround rounded projections of the groundmass which penetrate the crystal. Some of the broadest plates lie so that they show no twinning, indicating that they are broad by the large development qoPcxi. The angle of extinction is - 36° on either side the twinning plane, indicating anorthite.

Labradorite. - The feldspars of later consolidation, from to the smallest dimensions, extinguish with an angle of 12J° on either side the twinning plane, and so are very probably labradorite. They affect the lath shape more than the anorthite does. The larger are, however, broad, with square ends; the smaller, nan'ow rods; and they are rarely in contact, so that the ophitic structure characteristic of the common diabase is wanting.

The augite is also perfectly fresh, although the abundant inclusions, combined with the irregular cleavage, make it only imperfectly transparent. Separate crystals are bounded by perfect planes. An acute pyramid or dome appears with especial frequency. They are finely twinned, with the interposition of several very fine laminae at the center, and nearly colorless.

In one portion of the field is a group of the largest feldspar crystals (described above) loosely interlaced, and in the interstices the augite has developed so as to fill the spaces and produce exactly the basaltic structure. Only where the meshes were not closed do the augites project oiitward with terminal faces. If the crystallization had continued undistui-bed it would have produced a rock having exactly the texture of a Tertiary basalt. As it is, it is plain that the delicate featherwork of feldspar plates was floated along in the magma and its crystallization arrested, and one can see that the formation of the basaltic structure does not necessarily depend upon the crystallization, first, of the feldspar in a network of bars, and then, second, of the augite in the interstices thus left, but rather upon the different methods of the crystallization of the two minerals simultaneously; the augite, having started a crystal in contact with a feldspar blade, tended to increase this to a large, stout crystal, while the feldspar, rapidly shooting out new blades, inclosed and bounded the augite on all sides, hindei'ing the formation of crystal faces.

Hematite is quite abundant in the rock, blood-red, with botryoidal fibrous structure, showing the black cross abundantly, and aggregate polarization, and especially surrounding the inclusions of quartz and microcline. It also occurs, with blood-red color and fibrous structure, pseudomorph after olivine. The olivine form is sharp and clear, 2 P oo (021)^99° 15' (calculated 99° 06'), and this sharp dome is combined with the prism. When traces of the original mineral remain they extinguish longitudinally. Irregular cracks run across the mineral as in olivine and bound separate fibrous masses, the fibers being at right angles to the fissures. I conclude that an olivine very rich in iron - a fayalite - ^lias changed into hematite, more or less mixed with serpentine. The pseudomorphs are always surrounded by a whitish, semiopaque halo, probably from the silicic acid expelled from the primary mineral. Other similar forms are olive-green and polarize only in traces, and seem to be olivine changed to serpentine. They have also the white, opaque halo. Very rarely a rounded grain, which

The groundmass is made up of angular and rounded grains 0.001- across, which can at times be seen to be twins, and they seem to be, in part at least, augite, as they show an extinction at 42°. A specimen from the north edge of the dike has the large feldspars so filled by these minute augites that they occupy the whole space as closely as they do in the surrounding groundmass. Indeed, it appears as if a portion of the groundmass having a regular crystalline outline had been preserved intact from all decomposition, so that the interstices of the grains have not been filled with the fine dust of limonite, kaolin, etc., which renders the rest of the ground clouded. With polarized light the grains are seen to be optically orientated in the feldspar, as they extinguish together, and the feldspar bands can be distinctly seen shining through. The groundmass is for the most part the same in the portions included in the large feldspar crystals as outside, but some inclusions are red-brown and apparently glass. I can not detect with certainty any glass in the groundmass itself The inclusions are plainly from granite : quartz with sheets of pores, some containing moving bubbles, and rutile needles, microcline, centrally decomposed albite with extinction angle 4°, and orthoclase. Fragments of granite with feldspars wholly altered, and of an amphibolite quite fresh and closely resembling the fine-grained rock at the northeast corner of Amherst, also occur.

A specimen from the first cleared field north of the brook, externally hke the last, shows both the feldspars and the augite perfectly fresh and colorless, sharply defined, and distantly scattered in the ground. There is so much hematite that it takes up a considerable portion of the surface. One quartz inclusion is surrounded by a colorless radiated fibrous layer, and outside this by a broad band of hematite. The hematite so often sun-ounds the foreign inclusions in a rock otherwise fresh that one is tempted to assume it to have been a cement covering the grains before their envelopment in the lava.

A great number of bodies are present having exactly the shape of olivine crystals and a bright-yellow or red color. The yellow scarcely polarizes at all, some few fibers or isolated spots showing faint color, and it seems to be a yellow serpentine pseudomorph. The red shows a peculiar

Another specimen from the vicinity of the last has a very difiFerent structure. In a granular groundmass there are regularly disseminated, well-formed octahedra of magnetite, visible with a lens in the slide, and abundant diabantite-filled cavities. Small lath-shaped plagioclases and augites are distantly scattered and inconspicuous. The rock resembles that of the dike from the house south of the ruined leather mill below Mount Tom station, on the west of the river.

The above descriptions had been written before I received the first accounts of Mr. Diller's discovery of quartz-basalt at the Cinder Cone, in California. On sending him fragments of the rock here described, he wrote that the quartz resembled closely that of the Cinder Cone, and was more abundant.

It will be seen from my own descriptions that the idea that the quartz was original in the rock had not occurred to me. It does certainly resemble the Cinder Cone quartz very closely, and it is hard to see how a great quantity of foreign sand could be included in an erupted dike, and especially how it could fail to bring with itself moisture enough to make the rock vesicular. The shapes of the grains and the high greasy luster are not like granite-quartz. I have not been able to verify my observation that the slides contain microcline and mica, as the slides are not now a,ccessible, but the presence in the diabase, among many quartz grains, of a large fragment (5"'" across) made up of quartz and orthoclase is certain. The quartz was exactly the same rounded, bluish, greasy quartz as the rest, and the flesh-colored feldspar gave the optical tests of orthoclase, so that I feel quite certain that the unusual constituents have come in as foreign inclusions.

The structures produced by the introduction of this large amount of foreign material into the liquid trap resemble those described from the Greenfield bed in Chapter XIII (p. 419), where the sand has risen up into this lava from below. This locality was studied many years ago, before the Grreenfield and Holyoke beds were understood, and I can not say what modification of the above description might come from a new examination of the place with new light. The blue color of the quartz may be due to tension produced by heating, or the quartz may be derived from Algonkian blue-quartz gneiss. (See page 29.)

The Newer Cores And Suort Dikes The Tenth Core

The w()(mI road that runs north into the mountain west of Moody Corners branches after crossing the eleventh trap mass, and the western branch in a few rods runs out on the sands of the large Glacial lake described below. Here, at a pair of bars giving entrance to the field, near a small brook, begins a long outcrop of trap, which continues 50 rods west, forming the bluff which made the south shore of the lake. The coarse arkose surmounts it on the south, and it is by the downward pitch below the sands of this sandstone on the east and the west that the outcrop of the trap in those directions is limited, while the sands conceal its northern limit. Near its western end, where a stone wall runs across the sands, at the foot of a marked bluff, the sandstone resting on the trap can be seen to be well baked by it, and as the trap is wholly fine-grained and without steam holes it is plainly intrusive.

The Eleventh Or Black Rock Core

Looking southeast from the Mountain House, on the top of Mount Holyoke, one sees a prominent ridge of dark rock running parallel to the mountain - indeed, duplicating it on a smaller scale, repeating its easy southern slope and sharp northward-facing bluff and making with it the great sweeping curve. It differs radically from it in its origin, the larger deposit having been, as I have shown, a bed spread out over the subjacent sandstone, and this an injected dike cutting across the latter. (See PL IX, p. 446.) This bluff, as seen from the mountain, is called "The Black Rock," and I have chosen this name to designate the core, and also the whole series of the newer trap intrusions. Seen from the west side of the river above Smiths Ferry, it simulates exactly a volcano with sharp slopes and central depression.

The core is best studied at Batterson's quarry, in the northwest corner of South Hadley, near the last house (E. H. Lyman's) before the town line is reached. As seen in the accompanying view (PI. X), the nearly horizontal sandstones are a remnant resting with their edges against the diabase.

The latter not only cuts across the sandstone at this point, but sends into it apophyses of finer grain than the main mass, which have altered the sandstones in places for 4 feet from the contact and have fused themselves into firm union with the latter at their junction. The thin-bedded micaceous sandstones are delicately plicated by the intruded trap.

Following' the vertical wall of the diabase north 335 feet from the quarry - a wall which is the contact surface against wliich the sandstones formerly rested - one comes upon a most interesting point, where this wall is continued as sandstone, a fine contact being exposed, and the boundary line of the diabase and sandstone goes into the hill at a right angle. Climbing to the top of the bluff, one can follow this contact east, the sandstone at a distance of 4 feet from the diabase being baked into a dark-blue, hornstone-like rock. When the boundary bends round from east to north the thin-fissile sandstones have the imusual position, strike N. 70° W., dip 40° E., being thus thrown off from the eruptive rock. Continuing, the boundary returns westwardly, and thus embraces a great projection of the

For a long distance one can follow up the bed of Dry Brook I'unning on the back of the older diabase, while its left (south) bank is a vertical wall of sandstone dipping southward and ending abruptly against the diabase of the Black Rock dike, as indicated in fig. 27.

At the point where the first outcrop of the sandstone on the brook appears, about 590 feet from the contact in the vertical wall last described, occurs a curious metamorphosed limestone-breccia, with garnet, near the base of the sandstone. This nearness of the two diabase bodies continues, and one goes east a long distance thi-ough a valley with its right or north side till- back of the main diabase bed, rising gradually north to the Holyoke House, its left or south side the vertical wall of the Black Rock dike, its face veneered to a varying. distance upward with the remains of the sandstone.

When one comes out where one can look down on the cleared sand flats of the post-Glacial lake mentioned above, one sees that the boundary of the Mount Holyoke bed continues east, while that of the great crater swings round southeast and extends to the deep gorge of the little brook which drains the basin of the lake above mentioned, and has cut deeply through the diabase to enter Elmer Brook, just north of H. White's. The diabase continues to rise high and to carry a thin remnant of the sandstones in contact with its vertical face, which sandstone shows contact effects and can often be plainly seen to abut against and not to underlie the volcanic rock, toward which it dips. Wliere the boundary of the diabase runs southeast the sandstone preserved its east-west strike in the main, but in places dips toward the diabase with, the abnormally high angle of 80°.

At the west end of the mass the diabase appears in the road at the Lyman house, and its westward extension is concealed by sands. This is also the case with its southern border. The outcrop at the point where Elmers Brook ci'osses the road is so brecciated and its fissures are so filled with druses of small rhombohedra of hematite that it is probably near the southern contact.

On following the southern edge along to a point about north of the schoolhouse, where the road to South Hadley starts, it is seen that sandstones appear on the south of the trap, strike N. 65° E., dip 15° S. - finegrained, calcareous sandstones, blue-black as if baked or loaded with volcanic ashes, and rusting slowly inwardly, like the diabase, and between them and the diabase is a band, apparently 10 to 15 feet wide, of the most perfect tuff, made up wholly of angular trap fragments of the size of a pea, with here and there one as large as an acorn, all greatly decomposed. The exact relation of the tuff to the other beds could not be made out.

The boundary can be. closely followed eastward to the brook east of White's wood road, to which the northern boundary has already been followed. The exact contact can not be seen, but the dark rusting sandstones dip south away from the diabase, while the latter rock in the immediate proximity to the contact (6 to 8 feet distant) is compact, coarsegrained, and not porous.

Approaching the brook the boundary bends northeast, as that on the north side had bent southeast, and down the slope to the brook the diabase is amygdaloidal where nearest the sandstone. The boundaries have approached each other so that in the deep side of the narrow brook gorge only a fourth of the width of the great mass appears, but this is well exposed, and a great talus of fragments of a quite coarse diabase makes up the greater portion of its width. The bottom of this narrow gorge is covered with sand. In its opposite east wall one can trace from the south the southward-dipping tuffs, and from the north in fine cliffs the light-buff sandstones with the same dip, to where they approach the eastward continuation of the diabase ; and although the contacts are covered, it can be pretty plainly seen that from both sides the sedimentary rocks abut against the diabase. The latter is very fine-grained and has only a small fraction of the .width it had on the other side of the gorge. This gorge is 1 mile northwest of Moody Corners.

Going east it cuts through the tuff, and where this is coarse and both are decomposed it is very difficult to separate them. In one place the diabase is quite coarse, light-colored, and greenish from the abundance of diabantite, like that just east of White's wood road, and like the rock of the Deerfield bed at the Deerfield Notch. Followed still farther east, where it is crossed by the wood road north from Moody Corners, the diabase is on the north dark, fine-grained, and bounded on the north by sandstones which for a long distance east abut against the high wall of the diabase, as already described. Its boundary against the tuffs on the south is less clear. Where the road crosses, the distinctly columnar diabase rises in a ridge about 35 feet wide, and yet in this is a mass of tuff nearly a meter across, containing fragments of granite. To the south a narrow swamp separates it from a rock which seems to be a coarse volcanic agglomerate made up of angular fragments often 10 to 16 inches across, which in much-weathered exposures can hardly be distinguished from the normal diabase.

Petrographical Description

1. A section taken from the second outcrop by the roadside going in from Mr. Lyman's house and Batterson's quarry is the typical gray diabase, not distinguishable by the lens from the Iron Gate rock taken as a type above (page 461), and the microscope reveals little distinction between the two, either in structure or stage of decomposition. Pyrite occurs in excep-

The Newer Coubs And Shokt Dikes

2. In various sections cut from specimens taken at different distances up to 3,300 feet from the edge of the dike no distinction could be observed, but in one taken from very near the center the augites were in large, distinct crystals, very abundant, and plainly anterior to the feldspars.

3. Slides taken from the south edge of the dike, where Elmers Brook finally leaves the trap, showed a large development of the finely granular groundmass (grains so common in the rock of the tenth dike.

4. Sections were cut from the long, narrow, eastward prolongation of the dike where the Moody Corners wood road crosses it and at its intersection by the two roads next east. They resemble the type closely. The augite is in the main subsequent to the feldspars, but is a little more distinctly individualized in long blades. Olivine changed to an olive-green serpentine and distinct traces of the unaltered mineral occur sparingly.

5. In sections cut from the edge of the small apophyses sent off by the main mass into the sandstone and exposed in Batterson's quarry, we get additional proof that the larger feldspars are of earlier consolidation. These porphyritic feldspars are of the common size, 1 to across, and are associated with deep-green, well-formed olivines in an extremely fine-grained groundmass, so that it seems that they had already separated out in the magma before its injection into the narrow fissure in the sandstone, in which it cooled so rapidly that the customary ophitic structure was not produced but was replaced by the semicrystalline development described below.

The main groundmass is a felted mass of finest fibers 0.0016""" across, quite possibly feldspar microlites, which are not rigidly straight, but wavy, often beaded, and are clearly margarites ; generally, however, they polarize distinctly. These fibers have a radiated arrangement, which gives the whole groundmass a spherulitic structui-e. The fibers polarize sheafwise, although they are not parallel.

The presence of olivine in the fresh fine-grained diabase dikes in the granite, and especially in the minute dikes I have described (p. 416), as also its presence in the newer diabase of the volcanic plugs, particularly in that one which has been described as so full of quartz grains (p. 483), may seem, when contrasted with the absence of olivines in the great Deerfield and Holyoke beds, to indicate that the two former occurrences are to be associated together as of the same age and contrasted with the two latter and older occurrences; but olivine occurs in considerable abundance in the base of the Holyoke bed at the west foot of Provens Mountain in Westfield. It is strange, however, that where the inclusions of quartz are most abundant olivine is also most abundant.