Mineral Industries and Geology of Certain Areas - Vermont Agency ...

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80 REPORT OF THE VERMONT STATE GEOLOGIST. slate 4 feet wide and 10 feet deep. It is a picturesque and popular resort. Illustration, Plate XVII, shows the cascade, the eroded channel, and the rustic bridge above them. In the northeast corner of Thetford, about one mile south of Ely station and 50 rods west of the Passumpsic Railroad, there is an old quarry of extremely fissile, soft, argillaceous slate, with apparent strike north io° east, cleavage dip 85 ° east 10° south, strata (lip 45° north. It affords a most striking illustration of the nonconformity of planes of bedding and fissility. Crossing the entire exposed surface of ioo feet by 25 remarkably distinct lines of original bedding are seen. The cleavage planes are induced by subsequent pressure at right angles to the lines of fissility. About one-third of a mile to the northeast of Thetford Hill, at the old galena quarry, the cleavage is 40° east 30° south with no evidence of characteristic planes of bedding, unless the dip of the mineral vein, 21 ° to the southeast, affords us that evidence. It carries galena and sphalerite in considerable abundance. This Thetford argillite is a narrow band varying from onefourth of a mile in width, in Fairlee, to i mile in Norwich. It was not known to extend continuously north of Ely station until the author succeeded in the summer of '96 in tracing it through Fairlee and into Bradford, where the hydromica schist takes its place. The strike varies from north 10° east in Fairlee to north 50 ° east in Hartford. South of North Hartland it is north and south, and in Norwich north 45° east. The dip changes from 6o° west in Hartford to 85 ° east in Fairlee. With the general dip of the eastern flank to the west, or vertical, and a low dip of the planes of bedding just north and south of where the slate has been entirely eroded in Bradford, is sufficient evidence that it lies unconformably upoii the Huronian hydromica schist. The lapse of time between the deposition of the Huronian schists and the Lower Trenton slates was Ll I PLATE XVTT. Glen Falls, Fairlee.
REPORT OF THE VERMONT STATE GEOLOGIST. 81 long—sufficiently long for the former to have become the source of the latter. The chemical composition of the two is not too widely different. The ever active waters of the sea could easily wear away these older rocks, sift, redeposit the impalpable powder unconformably upon the Huronian rocks. The dip of the western flank is to the east, or vertical; but the planes of deposition of the slate have a low dip to the west, at Northfield 350 west. Along the entire line the evidence confirms an unconformity: 14 bands of slate, oulside of the two large belts, are interstratified with the Washington limestone. Unlike the belt upon the east, which is older than the Washington limestone and Bradford schist, the western belt is the youngest of them all and lies above them all. C. BRADFORD SCHIST. Bradford schist is the name which I have chosen for the noncalcareous member of the old " calciferous mica schist." The calcareous member is named Washington limestone from the town of Washington, Orange county, Vt., where it is extensively worked as a marble. Bradford schist is named for the town of Bradford, Orange county, Vt., where the non-calcareous element predominates. These, taken together, have hitherto been known as the " calciferous mica schist " of Professor Hitchcock, and no definite attempt had been made to separate the calcareous from the non-calcareous member until I took up the study of these ter - ranes in the summer of 1895. The larger part of the Bradford schist was originally a sandstone which is now represented by terranes of granular and micaceous cluartzite and a foliated mica schist. This is characteristically true in the eastern half of the county. The staurolite, actinolite and n-iagnetite schist of Vershire may have once been a cherty, iron hearing sand, infiltrated with carbonates of lime and magnesia, while some of the original calcareous sand deposits, somewhat destitute of lime, may
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