Part 14

CONGLOMERATE is consolidated gravel. Pebbles and boulders of all sizes are packed together by the stream which was moving them, and the spaces between the larger fragments are filled with the sand that settled in from the stream bed. The entire mass is cemented by silica, limonite, carbonates or some other substance deposited by percolating ground-water. The Devil's football near the Mount Holyoke House is a famous piece which was dislodged from the hillside above; and excellent specimens may be collected on Mount Toby, on Mount Sugarloaf, and in the cut at Mount Tom Junction.

ARKOSE resembles conglomerate, but the individual grains consist of mineral fragments, among which reddish feldspar is prominent. Quartz and mica may be present, too; and all the pieces are characteristically angular, commonly ranging from ^1/16 to 1/8 inch in size. The rock is red and crumbles easily. Beds of arkose alternate with conglomerate on the steep sides of Mount Sugarloaf.

GRAYWACKE is light to dark gray in color, and the fragments composing it are sand size pieces of older rocks. A few mineral grains, such as quartz, may be present, but mica is rare. Graywacke occurs interbedded with arkose in some parts of the valley.

SHALE is a thinly laminated sediment composed of microscopic quartz, feldspar, mica and kaolinite grains. Most shales in the Connecticut valley were deposited as muds in old lake beds. Some are red and record ephemeral pools, but others show from their black color, their coal layers, and their fish skeletons, that the water bodies in which they accumulated remained in existence for a comparatively long time.

LIMESTONE is a rock composed of calcium carbonate, and it consists essentially of an aggregate of calcite crystals or calcite fragments. It will give off gas bubbles in a very dilute solution of hydrochloric acid, and it exhibits other properties peculiar to the mineral calcite. A thin, sandy limestone bed has been identified in several sections of Holyoke.

The Igneous Rocks

Igneous rocks were once molten, and in this hot fluid state some were extruded at the surface as lava flows. Congealed flows reveal the motion, which brought them to their present resting places, in the banding and streaks that are so evident in the patterns of steam holes and minerals; but their massive structure bears witness to stagnation as they hardened. Other molten masses insinuated themselves into underground openings, where they solidified as intrusives, varying in size from small dikes less than an inch wide, to huge masses that can be measured in miles in any direction. Most of the igneous rocks in the highlands of western and central Massachusetts are massive intrusive types; light-colored varieties predominate, but some dark-colored dikes cut the older rocks both east and west of the valley. Dark-colored, massive and banded lavas are conspicuous in the ranges within the valley.

Igneous rocks may be divided into three general groups for practical classification, and each major group may be further subdivided. Rather conveniently each of the major groups may be recognized by the prevalent color of its rocks--whether dark, medium-colored, or light. And within each major classification there may be flows, characterized by banded structures and fine textures; small intrusives composed of well formed crystals in a fine-grained groundmass; and large intrusives consisting of goodsized, equi-granular crystals. Not all of these types can be found in central Massachusetts, but the variety of igneous rocks is surprising and offers some excellent possibilities for the collector.

The Dark Rocks

The dark rocks owe their color to iron-bearing minerals like olivine, pyroxene (augite), amphibole and biotite. All of these minerals weather to a rusty red surface, which is typical of their outcrops at many places.

BASALT is a black rock, dense in some places but perforated with bubble holes or vesicles, at others. It occurs throughout the length of the Holyoke, Tom and Pocumtuck Ranges; and fragments of basalt are abundant in the Granby tuff and agglomerate.

DIABASE resembles basalt but is distinguished by the thin, short crystals embedded in it. These crystals of labradorite feldspar resemble pieces of clipped thread, and they sparkle in reflected light. Almost all dark-colored dikes and the slowly cooled central portions of thick lava flows consist of diabase.

PERIDOTITE is a dark green, coarse, granular rock composed of olivine and subordinate amounts of pyroxene. It occurs near Westfield and Blandford, and at many places in Vermont.

The Medium-Colored Rocks

The medium-colored rocks contain approximately the same proportions of light- and dark-colored minerals. The dark iron-bearing minerals are relatively stable, but the light-gray feldspars decompose to kaolin and give the weathered rock a chalky white surface. Surface flows of this group are unknown in central Massachusetts, but the coarsely granular intrusives are well represented.

GRANODIORITE PORPHYRY is a greenish-gray rock occurring in many dikes in the western highlands. It has rectangular crystals of andesine feldspar up to 1/8 inch across, and these have a dull porcellaneous luster. A few dark-green amphibole crystals are only slightly smaller. Both feldspars and amphiboles are embedded in a very fine-textured, pale greenish groundmass.

GRANODIORITE is a gray equigranular rock containing flesh-colored microcline feldspar, white andesine feldspar, greenish flakes of chlorite, needles of amphibole and sparse grains of brown biotite. All crystals are about ^1/32 inch thick and commonly display a parallel arrangement. This rock forms huge irregular masses at Williamsburg, Whately and Belchertown.

The Light-Colored Rocks

The light-colored rocks are well represented by dikes and large masses but not by recognizable surface flows in central Massachusetts. Their exposures have rarely weathered much, because the predominant minerals are quartz, microcline, orthoclase and albite, which resist decay.

QUARTZ PORPHYRY is a light gray rock that is found in dikes. It has porcelain-white cleavable feldspars up to 1/8 inch thick, and dark glassy quartz of equal size in a granular mass of very fine-grained crystals. Intrusives of this type are numerous in the vicinity of Whately.

GRAY ALBITE GRANITE occurs in many dikes and small irregular masses throughout the highlands. All crystals have approximately the same size and rarely exceed ^1/32 inch in thickness. They consist of white orthoclase and albite, dark sugary quartz, and brown to black biotite.

RED MICROCLINE GRANITE is found in very large, irregular intrusives in the highlands. The crystals are ^1/16 inch or more in thickness. The red color is due to the flesh-colored microcline. Quartz is dark and glassy, and muscovite is the typical mica.

The Metamorphic Rocks

Metamorphic rocks were once sedimentary or igneous rocks which have been changed by intense pressure, by heat, or by solutions moving through them. Pressure usually produces a sheeted or foliated structure along which the rock exhibits a tendency to part--somewhat like the pages in a book that was bound before the ink was dry. Percolating solutions may produce chemical alterations in the original materials and even crystallize new substances along the foliated surfaces within the rock, much as water circulating through cooled soil may solidify to ice and cause heaving. Many of the rocks in the highlands bordering the Connecticut Valley are highly foliated or banded in consequence of the mechanical deformation they suffered when the ancient upland mountain system was created. They include the slates, schists and gneisses. A few massive types, like marble, serpentine and soapstone, owe their origins chiefly to the effects of heat or of the hot, chemically charged solutions which permeated them.

SLATES are fine-grained rocks characterized by flat, parallel cleavage surfaces which usually cross the original sedimentary structure. They were formed from shales, by shearing and compression during ancient mountain-making movements. Slates crop out beside the station platform at Brattleboro, Vermont, and at many places southward along Federal Highway 5 to Greenfield.

SCHIST is foliated, too, but it is composed largely of cleavable minerals, such as chlorite, muscovite, biotite and amphibole, which are distributed along the cleavage surfaces. These minerals result from the chemical activity of hot solutions circulating along a slaty cleavage, re-crystallizing old materials, and bringing in new to make these coarse mineral flakes. The schist receives its specific name (biotite schist, chlorite schist, etc.) from the mineral which accentuates its cleavage structure.

A few schists contain large crystals which bulge the schistose surfaces outward around them. Garnet is characteristic in this role, and a muscovite schist with garnets in it is called a GARNETIFEROUS (or garnet-bearing) MUSCOVITE SCHIST. Other minerals with occurrences similar to the garnet are microcline, albite, staurolite, amphibole, tourmaline, pyrite and magnetite.

GNEISS is a banded rock containing cleavable minerals, but it lacks the cleavage structure of schist. The cleavable minerals (biotite, muscovite, amphibole, etc.) may give the gneiss its specific name, but as often as not, the name is derived from the whole mineral assemblage, or from an assumed origin, as in the case of granite gneiss. As in the igneous rocks, the mineral ensemble is held together by interlocked quartz and feldspar grains. Black-banded biotite gneiss and hornblende gneiss are the most abundant varieties in the neighborhood of the metropolitan reservoir east of Pelham.