22 Report of the State Geologist. 



crops of Potsdam sandstone are more numerous close to the boundary in this 



township than in any other part of the district examined. 



The gneisses along the boundary in Hopkinton are quite homogeneous 



and quite similar to those already described, their main distinction being the 



not infrequent occurrence of considerable biotite, this being especially true of 



those in the western half of the town.* In the eastern half they are mainly 



well foliated orthoclase or microcline gneisses with the usual variations in 



structure and texture and in the amount of quartz present, containing also 



varying amounts of plagioclase and hornblende and sometimes biotite. The 



usual bands of diorite gneiss occur plentifully. In an exposure on the Meacham 



* * 



farm, two miles south of Mcholville, is a dike-like band only 18 inches wide 



cutting the gneiss, and this furnishes the best and least metamorphosed speci- 

 mens of the basic, ophitic gabbro which the writer has yet seen in the 

 Adirondack region. 



The Potsdam, as exposed in the township, possesses considerable interest. 

 Near the roadside at the Meacham farm and near the gabbro just discussed 

 are the outcrops mentioned on a previous page, f a low knoll of very coarse, 

 rotten, acid gneiss crossing the road, followed to the south at a distance of 15 

 yards by exposures of a coarse, feldspathic conglomerate which disintegrates 

 with great readiness. The conglomerate is composed of debris from the 

 gneiss and occupies a depression in its surface, as gneisses appear again in 

 force a short distance further south. 



One mile to the eastward, on a lane running south from the main road,, 

 is an old quarry opening in a single ledge w^hich protrudes through the drift. 

 The rock is a well laminated, coarse sandstone, in white and buff or brown 

 colors, and has the abnormal dip of 18° to S. 20° E. It seems to represent an 

 horizon well above the base of the formation and its attitude suggests disloca- 

 tion. It lies, however, in a wide depression with no other outcrops near 

 at hand. 



One and one-half miles west of the Meacham farm exposures and less than 

 two miles south of Hopkinton village, a ledge of red, feldspathic sandstone, con- 

 taining also much magnetite sand, is well exposed by the roadside and for some 

 distance to the westward, with the normal low dip to the north-west. Only 

 ten yards south of it and parallel with it on the west, is a ridge of very coarse 

 acid gneiss. There is no evidence of faulting between the two. One mile to 

 the northward is Budd's quarry wdiere a firm, red stone, often prettily banded 



* As Professor Smyth is engaged in a detailed study of the Pre-cambrian rocks of St. Lawrence county, they will be but 

 briefly referred to here. 

 t Page 14. 



