REPORT OF THE VERMONT STATE GEOLOGIST. 47 IC) F C) I- C) 0 C) 0 0 0 C) 0 C) C) The key to this topography is to be found in the disposition
REPORT OF THE VERMONT STATE GEOLOGIST. 49 48 REPORT OF THE VERMONT STATE GEOLOGIST. open at this point, therefore, <strong>and</strong> the stream must have been subglacial. General geology; Pctrograpliy <strong>of</strong> the schists.—The schistose <strong>and</strong> slatv members <strong>of</strong> a series <strong>of</strong> metamorphic rocks are found over almost the whole <strong>of</strong> Barre township, underlying the glacial drift, <strong>and</strong> appearing in frequent outcrops through it. The area occupied by these rocks is indicated by the unshaded portions. <strong>of</strong> the map. They are older than the granite, as being the country rock into which its mass was intruded. They show considerable uniformity in field habit, <strong>and</strong> in their areal relations. They are prevailingly slate colored or dark gray rocks, generally with a well developed cleavage <strong>and</strong> the shimmering appearance due to abundant minute flakes <strong>of</strong> mica. By differ - ences in mineralogical composition an alternation <strong>of</strong> excessively fine lighter <strong>and</strong> darker b<strong>and</strong>s is produced. The larger <strong>of</strong> these differences, between the portions rich in lime <strong>and</strong> those high in silica, may at times be traced continuously for several hundred yards. They afford the best means for observing the areal relations <strong>of</strong> the schist. The strike is found to be uniformly northwest <strong>and</strong> southeast. The dips are high. Very frequently the rocks st<strong>and</strong> on end. At times they show much contortion by pressure over short distances, as in the railroad cuts to the southwest <strong>of</strong> the city. Here numerous lenses <strong>of</strong> quartz are found, which elsewhere in the series are not so well developed. TJsuallv, however, the schists are not greatly disturbed. They present the appearance <strong>of</strong> metamorphosed sediments, but their sedimentary origin can not be set down as proven. No chemical analyses have been made which might <strong>of</strong>fer the means <strong>of</strong> deciding between their possible igneous or sedimentary originals. No occurrence <strong>of</strong> marble is met with in the metamorphic series throughout the area studied. No unalteredi sediments were found in it which could be traced continuously over to their metamorphosed representatives. Still in the field the rocks present an appearance as <strong>of</strong> sedimentary beds, but little disturbed by metamorphism from their original relations,, <strong>and</strong> when seen under the microscope they are found to contain the minerals which would be furnished by limy shales <strong>and</strong> impure s<strong>and</strong>stones. The study <strong>of</strong> their sections shows that the minerals commonlY present are quartz, biotite, calcite, <strong>and</strong> magore or less feldspar is usually found. Quartz grains, n etite. M fractured <strong>and</strong> angular, <strong>and</strong> magnetite, withccasional o large flakes <strong>of</strong> biotite, <strong>of</strong>ten make up a large portion <strong>of</strong> a single b<strong>and</strong>. Again a member <strong>of</strong> the series, two or three feet in thickness, will be found which is very rich in calcite <strong>and</strong> low in quartz. Magnetite <strong>and</strong> biotite in greater or less amounts are invariably present. The series <strong>of</strong> schists is cut by dikes <strong>of</strong> pegmatite <strong>and</strong> camp tonite. The dikes or veins <strong>of</strong> pegmatite are in two kinds, those which have the mineralogical composition <strong>of</strong> granite <strong>and</strong> those onsistiflg entirely <strong>of</strong> milky quartz. The former are found near h quartz is the only mineral the granite mass, w hile those in whic present occur at a distance <strong>of</strong> a mile or more from any <strong>of</strong> its outcrops. They represent, in all probability, the secretion from the parent magma at the most distant points along dikes <strong>of</strong> pegmatite. One <strong>of</strong> these quartz dikes, in the western part '<strong>of</strong> the township <strong>and</strong> southward from Barre, is three feet in width. The most i nteresting <strong>of</strong> the normal pegmatites is the dike which occurs in the brook course along the road leading toward East Barre, two miles distant from Barre itself. It is about eight inches wide. The minerals present are feldspar, quartz, muscovite <strong>and</strong> tourmaline. The walls <strong>of</strong> the dike are nearly parallel with the beds <strong>of</strong> e nclosing schist, <strong>and</strong> both are vertical. The incoming <strong>of</strong> the pegmatitic material has loosened plates <strong>of</strong> the schist, so that at one end they are still connected with the wall-rock, but are warped back at their distal extremities. They have taken the easy curve which is produced by a knife passing along a block <strong>of</strong> wood, but not detaching a chip wholly. These partly released <strong>and</strong> partially included portions <strong>of</strong> the wall-rock are found ex tending in either direction along the dike. They are about i8 cm. in length by .5 cm. in thick- It has been pointed out to the writer by Dr. W. D. Matness. thew that the mere effects <strong>of</strong> heat might account for this warp-
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