Gushing — Potsdam Boundary. 13 



less load than at the time of their great metamorphism, and thus a long inter- 

 vening erosion period is suggested. 



The erupted rock may or may not have reached the surface. So far as 

 known it is found at the present day solely in dikes. These are exceedingly 

 abundant in the eastern Adirondacks. Though presenting considerable varia- 

 tion they may all be classed as diabases, and mostly as olivine diabases. 



Accompanying these dikes, and having, so far as observed, the same area 

 distribution though far less abundant, are other dikes, ordinarily of red color, 

 of a much more acid rock. The writer has heretofore classed these with 

 the trachytes (bostonites), but they present constant differences when com- 

 pared with the lake Champlain bostonites, and evidence is accumulating that 

 they are distinct in age. They are quite numerous in Clinton comity and in 

 the eastern part of Franklin, and have not been seen cutting any but the Pre- 

 cambrian rocks. While they may be nothing but representatives of the post- 

 Utica trachytes the writer is disposed provisionally to regard them as 

 distinct, and of Pre-cambrian age. They have the same general east and west 

 trend as the diabases, and neither has as yet been observed cutting the other. 



VIII. Palaeozoic Jiocks. After this period of dike formation, erosion con- 

 tinued in progress for a considerable length of time. Then ensued a depres- 

 sion carrying all the peripheral portion of the Adirondack region below sea 

 level, where it remained during the deposition of the Potsdam sandstone and 

 the lower Silurian limestones, which were laid down on the deeply denuded but 

 uneven floor of the older rocks. 



The Potsdam sandstone north of the Adirondacks is of very considerable, 

 though unknown thickness. It is at least as much as 500' however, and 

 probably considerably more. From the lack of fossils except in the upper por- 

 tion it is an extremely difficult formation to subdivide. The extreme basal 

 portion is a conglomerate, often very coarse, and also carries layers of coarse, 

 feldspathic and hematitic, easily rotting sandstone. Otherwise it is a quite 

 pure quartz sandstone, though occasionally some layers are dolomitic. The 

 basal one-fourth is prevailingly of red color, while the remainder is white, 

 yellow or brown. It grades into the Calciferous dolomites above throng] 1 

 passage beds, 30' to 50' thick, of alternating layers of sandstone and grey 

 dolomite. In its upper portion it carries an upper Cambrian fauna. 



IX. Post-Uiica Uplift. After the close of the lower Silurian the sub- 

 merged district was raised anew above sea level and was then affected by the 

 earth movements which caused the Green mountain uplift. But whereas the 

 rocks of Vermont were folded, faulted and metamorphosed by their action, the 



