GRANODIORITE PORPHYRY is a greenish-gray rock occurring in many dikes in the western highlands. It has rectangular crystals of andesine feldspar up to ⅛ 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 ⅛ 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.
MARBLE is a granular rock composed of calcite crystals. It is formed when heat volatilizes the bituminous coloring agents of ordinary limestone and simultaneously causes enlargement of the calcite grains. It is the principal rock in the Berkshire Valley in which North Adams, Adams and Pittsfield are located.
OPHICALCITE is a lime-silicate rock. It is formed by the chemical reactions of hot solutions on limestone or marble at considerable depth within the earth. The original calcite is converted into diopside, garnet, vesuvianite and tremolite, forming a rock that may be massive, or which may preserve some of the original bedded structure. It is found in association with the crystalline limestone and magnetite at the old iron mine, located one mile north of Bernardston.
SERPENTINE is a dark-green rock made almost exclusively of the mineral serpentine. It results from the reaction of hot solutions on olivine and pyroxene rocks (peridotites). Serpentinite is present in the Westfield marble quarry and at Zoar on the south side of the Deerfield River.
SOAPSTONE is composed principally of talc. It, too, results from the chemical activity of hot solutions ascending through serpentine and causing the mineral transformation. Bodies of this material are associated with the serpentinite at Westfield and Zoar, and northward in sections of Vermont. It is mined for talc, but in colonial days it found many uses. The colonists used cross-cut saws to make blocks for foot warmers in their sleighs, to control the heat in the old wood-fired ovens and to make water pipes before iron and lead were available in adequate quantities. Many soapstone articles may be seen—and purchased—in Wiggins Country Store and in other good antique shops through the valley. One of the most primitive Indian cultures in this region utilized soapstone pots, and exhibits are on display at both the Springfield Museum of Natural History and the Amherst College Museum.
Conclusion
To anyone who has had the patience to read through the preceding pages and to reach these concluding remarks, it must be obvious that geology is not merely a pastime for specialists. It does not take half a dozen college and university degrees to collect rocks and minerals, and to understand what they mean; or to appreciate not alone the beauty, but also the long and involved, yet logical, origin of scenery; or to comprehend from a rock-cut or cliff the vast changes which have occurred in the course of geologic time; or to grasp the current significance, as well as the historical importance, of such rock and mineral products as the trap, the limestone, the pyrite, the lead veins, the soapstone, the varved clay, the gravel banks.
Whether one’s interests are practical, historical, acquisitive, esthetic, philosophical or scientific, the geological features of the Connecticut Valley possess the variety to gratify them all. One must indeed be blind if he cannot find something of interest—a hobby—even a profession in the geological display spread before him in central Massachusetts. Let it not be thought that this little volume tells the whole story. On the contrary, its authors expect to have a difficult time justifying their sins of omission, more particularly because many of the omissions have been conscious and deliberate. But they trust they have left for the reader a wealth of features which he can make his own by right of discovery. For it will not take him very long to penetrate the fourth dimension of geologic time more deeply and intimately than is possible in the pages of a book.