GEOLOGY OF THE LAKE PLACID QUADRANGLE 97 



strata preceded, or possibly was in part at least concomitant with, 

 the great intrusions ; and that the foliated character of the intrusive 

 rocks is essentially a flow-structure developed under moderate 

 pressure during late stages in the consolidation of the magmas. 



The great Precambrian land mass just referred to was above 

 sea level, and underwent weathering and erosion for some millions 

 of years at least, extending through later Precambrian time and 

 into the early Paleozoic era, as proved by the facts that the oldest 

 rocks deposited upon the Precambrian floor are of late Cambrian 

 age, and that the rocks of this Precambrian floor immediately 

 below the Cambrian strata exhibit textures and structures which 

 could not possibly have been produced except at very considerable 

 depths below the earth's surface. 



Following the anorthosite and syenite-granite intrusions and, for 

 most part at least, during the long time of erosion above men- 

 tioned, came the minor intrusions of gabbro-diorite, gabbro, peg- 

 matite and diabase. Of these, the diabase is the youngest with the 

 finest grained texture showing a cooling of the rock comparatively 

 near the surface of the earth. In the Adirondack region, pegmatite 

 dikes commonly cut the gabbro and hence are younger. Whether 

 the gabbro bodies are younger or older than the gabbro-diorite dikes 

 of the Lake Placid quadrangle could not be positively determined 

 but they are probably younger. 



Paleozoic History 



By late Cambrian time the profound erosion above referred to 

 had worn down the whole Adirondack region to a comparatively 

 smooth, low-lying (peneplain) surface. This is proved by the fact 

 that the late Cambrian strata (particularly the Potsdam sandstone), 

 which are the oldest to have been deposited upon the Precambrian 

 rocks, everywhere rest upon a peneplain surface of the latter. In 

 the northeastern Adirondacks, including the Lake Placid quad- 

 rangle, this peneplain was moderately rough with some hills rising 

 probably several hundred feet above the general level, but of course 

 this was not at all comparable to the high, rugged relief of the 

 present day. 



The best available evidence indicates that the ancient peneplain 

 became sufficiently submerged during late Cambrian time to allow 

 the sea to cover all but a considerable part of the central Adiron- 

 dack area, and that the maximum submergence occurred during 

 mid-Ordovician time when a still smaller part of the central Adi- 

 rondack region remained as a low island. Judging by the marine 



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