450 NEW YORK STATE MUSEUM 



amount of surface wear during the interval. The thickness of rock 

 removed is purely conjectural but must have been large, several 

 thousand feet at least. 



Later igneous rocks. Toward the close of this long erosion period 

 came another time of igneous activity in the region, molten rock 

 ascending toward the surface, and utilizing a system of east-west 

 fissures for its ascent. Such lava-filled fissures are known as dikes, 

 and such dikes are very numerous in the northeastern Adirondack 

 region, though rather uncommon in the district under considera- 

 tion. There was likely volcanic action at the surface, but this can 

 only be conjectured since no known vestige of that surface now 

 > remains, all having been since worn away. The source of the ma- 

 terial is equally conjectural, though quite likely the same as that 

 whence the great intrusions sprang. At the present surface we 

 see only the old, lava-filled channels of ascent. 



Erosion still continuing after the close of the igneous activity, 

 the surface was still further lowered, but by an amount to be meas- 

 ured in hundreds rather than thousands of feet, the character of 

 the dike rocks clearly indicating that they solidified at no great 

 depth. 



Paleozoic submergence. Around the borders of the Adirondack 

 region we find, resting upon the Precambric rocks, a series of 

 sandstones, limestones and shales of early Paleozoic age, the Pots- 

 dam sandstone of Cambric age beneath, and above in order the 

 Beekmantown dolomites and limestone, the Chazy, Lowville, Black 

 River and Trenton limestones, and the Utica shale, all of Lower 

 Siluric age. In the heart of the region such rocks are wholly 

 absent, save as scattered glacial boulders. Yet nothing is more 

 certain than that they formerly extended over much of, if not 

 over the entire, Adirondacks. When the submergence beneath 

 the waters of the sea began, the region had been worn down to a 

 comparatively smooth surface by long-continued erosion, and 

 seems to have had a low, domelike summit in the present south- 

 western part of the region, whence it sloped gently away on all 

 sides. The encroachment of the sea was not steady but in oscil- 

 latory fashion, but was in general progressive ; in other words the 

 waters of the successive seas usually covered a larger part of the 

 dome than their predecessors had done. This was especially true 

 in the northeastern part of the district. Where we today find the 

 Paleozoic rocks we can be sure that the sea was present, but since 



