Part 13

MICROCLINE is a white to flesh-colored feldspar with two almost perpendicular cleavages. It will scratch glass or a knife. One cleavage face shows a grid of translucent and transparent lines intersecting at 90°.

ALBITE is the second most abundant feldspar. It is white and may generally be recognized by its two cleavage surfaces at 86°. Its growth may be likened to piling a series of plates with their surfaces parallel to one of the cleavages; during growth the plates are laid alternately face up and face down, so that the 86° cleavage edges zigzag in and out, forming a surface which, on the average, is perpendicular to the growth cleavage surface. The separate plates can usually be detected as fine bands or striations. The mineral scratches either glass or a knife.

MUSCOVITE is the white mica found in tabular crystals that can be cleaved into flexible and elastic sheets. It can be scratched and cut easily with a knife or shears.

BIOTITE is an amber-colored to black mica. Like muscovite it is flexible and elastic, but it is slightly more brittle.

TOURMALINE crystals occur in triangular prisms with the corners bevelled so as to give them a rounded appearance. They lack cleavage, are very brittle, and will scratch glass. Black is their usual color, but red and green varieties are present in many pegmatites.

SPODUMENE crystals are white to pale rose in color, and they occur as flattened prisms with bevelled corners. They cleave parallel to the surfaces bevelling the corners. The mineral is much harder than a knife, and the cleavage surfaces have a lustrous, slightly satiny appearance.

RADIOACTIVE MINERALS occur in many pegmatites and metamorphic rocks of this region. The species which have been formed as a result of recent alteration are brilliant golden or green encrustations in cracks or on a pitchy-black nucleus. The most abundant ones are uranite, autunite and torbernite. Older primary or source minerals are pitchy-black and are surrounded by a narrow rusty red zone or "halo," ^1/16 to 1/8 inch wide; an elongate species resembling a rusty hand-made nail is allanite; the more pitchy, irregular-shaped mineral is usually uraninite or pitchblende.

Other minerals found in pegmatites in the Connecticut Valley region include beryl, apatite, zircon, garnet, fluorite and lepidolite.

Most of the minerals in normal igneous rocks are too minute to be recognized easily, but a few have distinctive characteristics which serve to identify them. QUARTZ is a hard, dark, glassy-looking mineral without cleavage. ORTHOCLASE and MICROCLINE feldspar are hard, flesh-colored (occasionally white) minerals with flat cleavage surfaces. The minerals making the white lathlike mosaic on the weathered surface of the Range at the Mount Holyoke House are LABRADORITE feldspar. They are about ¼ inch long and ^1/50 inch thick--too small to permit testing by ordinary physical methods, although unweathered pieces have essentially the same physical properties as orthoclase and microcline. The MICAS are flaky and reflect light like minute pieces of tinfoil; muscovite is white, and biotite is amber-colored to black. CHLORITE resembles mica but is less lustrous and is dark green.

Some minerals of igneous rocks do not appear in pegmatites. Among them is OLIVINE, which has almost the same color as chlorite but is harder than a knife and is massive or granular. It is commonly associated with massive green SERPENTINE, which is softer than a knife. These three minerals are especially abundant in rocks found in the vicinity of Blandford, Massachusetts, and Dover and Chester, Vermont.

AUGITE is a dark brown to black pyroxene which occurs between the mosaic of whitish labradorite feldspar prisms in the weathered diabase near the Mount Holyoke House.

AMPHIBOLE crystals are dark green to black, "match-shaped" crystals. They have almost the same hardness as a knife and are characterized by two cleavages parallel to their length and intersecting at 56°. The mineral is also abundant in metamorphic rocks and is frequently reported as a "fossil fern" from ledges at Charlemont and Shelburne Falls.

Minerals of Metamorphic Rocks

The principal minerals of metamorphic rocks include many which are likewise present in pegmatites and igneous rocks, such as microcline, albite, quartz, muscovite, biotite, amphibole, serpentine and tourmaline. But there are others which are more exclusively metamorphic:

GARNET occurs in twelve- or twenty-four-sided red crystals. It is much harder than a knife. The geometric form is diagnostic, and crystals up to an inch thick are obtainable in Plainfield, Massachusetts, and at Grafton, Chester, and Gassetts in Vermont. They occur in a muscovite schist, in which the muscovite flakes are wrapped around the individual crystals.

TALC is a white to pale-green mineral found around the margins of intrusive rocks that are rich in olivine and serpentine. It is foliated and is so soft that even solid masses will rub off on cloth. It is present in the green marble quarry near Westfield.

KYANITE is a sky-blue, bladed mineral, with two excellent cleavages at nearly 90°, and a good smooth fracture at almost 90° to both. One face is harder than a knife and the other two are softer. It is very abundant in the country rock southeast of the Westfield marble quarry.

The Minerals of Soils and Rock Decay

Aluminous minerals decay to KAOLINITE, and those with a high iron content alter to LIMONITE. Both these products of decomposition form a sticky paste in their original forms. Kaolinite is white to yellow, and limonite or ochre is yellow to orange. Limonite also appears in orange-colored or brown balls, in icicle-like masses, and in thin beds. Specimens have approximately the hardness of a knife. Quartz does not decay easily and remains behind in solid granules.

The Minerals of Sedimentary Rocks

Most sedimentary rocks are formed by the cementation of deposits of transported waste, derived from older materials. They may contain anything. The minerals which undergo rapid decay break down to limonite, kaolinite and quartz, leaving only the more resistant varieties, which include, in order of decreasing resistance, quartz, microcline, orthoclase, albite and muscovite. Less abundant constituents are garnet, tourmaline, zircon and magnetite.

Certain kinds of sedimentary rocks may be formed through other agencies--for example, limestone, which is composed of calcite, initially precipitated by lime-secreting organisms or by the evaporation of lime-charged waters. The effects of organic activity may be seen in the limestone near Bernardston, but most of the calcite now present in the rocks of western Massachusetts is of vein or metamorphic derivation. Salt (halite) and gypsum are formed by the evaporation of saline waters, but only the vacated casts of salt crystals have been detected in the Triassic sediments of the valley.

THE ROCKS

Rocks record three distinct methods which nature employs in the aggregation of minerals. The sedimentary rocks register the work of wind, water and ice. Deposits left by wind and water are generally stratified or bedded, and they, together with glacial deposits, are composed of fragments which touch one another and are cemented at the points of contact. Igneous rocks record the solidification of hot liquids which injected themselves into older rocks or filled crevices, and which, upon cooling, formed masses of closely fitting crystals. The third group includes types which are crystalline like the igneous rocks, and which may be laminated somewhat like the sediments; they show effects of heating and squeezing until their original forms and even their minerals were changed. These are the metamorphic rocks.

Anyone who wants an orderly record of geologic history will arrange his rocks into these three groups--the sedimentary, the igneous, and the metamorphic. In the Connecticut Valley the metamorphic rocks reveal the ancient phases of earth history, and the sediments contain the details of younger or later geological episodes. The igneous rocks have a wider historical range; and, like the other types, they record a long period of violence and upheaval which seems out of harmony with the placid countryside for which they now provide a solid foundation.

The Sedimentary Rocks

The sedimentary rocks are built from the disintegrated wreckage of older ones. The products of rock decay are picked up and dragged, or carried in suspension or solution, by wind, running water, or moving ice. They are deposited when and where the transporting agent can no longer function. Such rocks are usually layered because the transporting power of the carrying agent fluctuates. Bands of one kind of material, separated by dissimilar materials above and below, are called beds.

The bedded or stratified rocks of the Connecticut Valley vary greatly, from the coarse bouldery deposits in Mount Toby to the fine-textured, red and black laminated beds at Whittemore's Ferry. Conglomerate, arkose, graywacke, shale and even limestone are represented, but there is little true sandstone. Sandstone is an even-textured, granular rock, most commonly composed of cemented quartz grains. Its uniformity of grain-size and composition reflects prolonged weathering of the original rock and good sorting of the fragments as they were transported to their new resting place. The sequence of exposure, transportation and deposition was too rapid in the ancient Connecticut Valley to permit appreciable decay and sorting; hence sandstones are absent. Limestones and salt beds are likewise rare, but the metamorphosed limestones which are found in the western highlands and in the Berkshire valley demonstrate that limestone-forming processes played a significant, if intermittent, part in the history of the region.