PHYSIOGRAPHY OF THE EASTERN ADIRONDACK^. 409 



sandstones were deposited along: an encroaching shoreline.* The commonest Pa- 

 leozoic outlier now remaining in the interior of the mountains belongs to the Pots- 

 dam, but Calciferous limestone is met in the most remote one of all here specially 

 described, so that it is probable that at least the Trenton limestone and the Utica 

 slate were also deposited in the valleys later described. It should be appreciated, 

 however, that on the southwest side of the Adirondack crystalline rocks the Tren- 

 ton limestone extends farthest into them, having apparently buried the earlier 

 strata. This may indicate a different amount of submergence between the two 

 sides of Adirondack island. t 



Previous Work bearing on the Problem 



Hitherto attention has been chiefly directed to the progress of events on the sea- 

 bottom, but in pursuing the question further it is also interesting to inquire what 

 were the conditions prevailing over the land area during these times. The crystal- 

 line rocks had all been formed and metamorphosed before the Cambrian opened. 

 The igneous rocks had largely been squeezed into gneisses ; the limestones had be- 

 come coarse, graphitic marbles ; and very quartzose, but not abundant, sillimanite 

 gneisses indicate sandstones. The nature of the other acidic gneisses is yet in 

 doubt. Unmetamorphosed pre-Potsdam diabase dikes have been shown by H. P. 

 Cashing t which represent this pre-Cambrian or early Cambrian interval. During 

 the early Cambrian the Adirondack height-of-land must have been subjected to 

 the ordinary processes of erosion and land sculpture, and it is the purpose of this 

 present paper to show the traces of their work that are still discernible. The old 

 topography has been greatly masked by later faults, in which the Paleozoics have 

 shared, but enough in the way of remnants and outliers of unaltered and almost 

 horizontal Cambrian and Ordovician sediments remain to be of real significance. 



Influence of Limestones on Topography 



In directing the early as well as the present lines of drainage, which latter prob- 

 ably in large part adhere to the early ones, the coarse, crystalline limestones had 

 a very large influence. They may not be thick, continuous, or extensive as regards 

 individual beds, but in the aggregate they were very serious factors, for they are 

 to be detected in almost all the large depressions. 



Keeshville Embayment 



Just south of Port Kent a range of crystalline rocks juts out into the lake as the 

 bold headland of Trembleau point. Around its northern portion the Potsdam 

 sandstone forms an embayment that reaches to Keeseville and that furnishes the 

 walls of the interesting gorge of the Au Sable chasm, familiar to so many visitors 

 to the region. From Keeseville the line of the Potsdam bears away to the north- 



* See in this connection C. D. Walcott, Bulletin 30, U. S. Geological Survey, 1886, p. 24 and fig. 2. 

 Mr. Walcott even suggests the former presence of (Upper) Silurian strata over the Adirondack 

 crystallines, but beyond this reference to them he does not discuss their physiography. 



tSee the recentlx- issued geological maps of New York state, the large one by James Hall and 

 W J McGee in 1894, and the small one, which is as regards the eastern Adiroudacks more accu- 

 rate, by F. J. H. Merrill, in Bulletin New York State Musuem, vol. iii, number 15, 1895. 



i H. P. Cushing : On the Existence of Pre-Cambrian and Post-Ordovician Trap Dikes in the 

 Adirondacks. Trans. N. Y. Acad. Sci., vol. xv, 1896, p. 248. 



