REPORT OF THE DIRECTOR I917 I23 



through a contact phase of the syenite. The excavation and lev- 

 eling of Broadway has exposed it at the side of the street. It is 

 about 5 feet wide and runs nearly northeast-southwest. Still 

 another is to be noted i| miles south of McCauley pond, beside 

 the road, not far from the Gabbro knob. Only two dikes are reported 

 in the region about Long lake, 1 while in the eastern section the 

 country is fairly seamed with these black bands. 



In composition, the dikes are somewhat similar to the gabbros 

 but the textural make-up is far different. 



These dikes must have welled up nearly if not fully to the surface, 

 for they display a striking chill effect. Close to the point of con- 

 tact with the country rock the diabase dikes are very dense and are 

 actually glassy in nature, showing that they were cooled too rapidly 

 to allow complete crystallization of the minerals. Farther from the 

 edge the glassy constituent is less and less prevalent until in the 

 center of the wide dikes, some of which are 30 or 40 feet in thickness, 

 the rock is wholly crystalline. The ridge known as the Palisades 

 of the Hudson, so familiar to New Yorkers, is a wide dike (or more 

 properly a sill) of diabase similar in composition to the Adirondack 

 type, although of very much later age. Along the shore of Lake 

 Champlain dikes of later age and of syenitic composition occur, 2 but 

 we need not discuss them here. 



The Later Sedimentary Rocks 



With the dying out of the igneous activity the region was again 

 subjected to the ravages of water, frost and winds, subduing the 

 raggedness of its topography. The land experienced a general 

 lowering and slowly sank below the waves of the sea, when a period 

 of rock construction succeeded the ages of destruction. Around 

 the shoulders of the Adirondacks layer upon layer of water-laid 

 sediments were deposited, which, as time went on, became rocks. 



The Potsdam Sandstone 

 The first deposit was a conglomerate composed of the refuse of 

 the wear upon adjacent land areas. This is known as the Potsdam 

 basement bed. Resting upon this a purer sandstone is found. 3 The 



1 H. P. Cushing, N. Y. State Mus. Bui. 115, p. 484. 



2 Kemp, J. F. & Marsters, V. F., The Dikes of the Champlain Valley, U. S. 

 G. S. Bui. 107. 



3 Recent work seems to indicate that these beds are of different age from 

 the Potsdam, but this is of small moment to us here. 



