PLASTIC DEFORMATION OF GRENVILLE LIMESTONE 



BY D. H. NEWLAND 



Some of the most interesting exposures of the Grenville Ume- 

 stone in the Adirondack region are found along the shores of Lake 

 Chamiplain, on the New York side, where the series of ridges that 

 confront the lake have been cut through to afiford room for the 

 tracks of the Delaware & Hudson Railroad. The rock faces still 

 preserve their fresh appearance in contrast with the stained and 

 weathered condition of the limestone in natural outcrop ; and the 

 vertical sections afford opportunity for observation of the rock 

 in mass, with its complicated structures and deformation 

 phenomena. 



The accompanying series of photographs, contributed by Prof. 

 G. H. Hudson, reveal the conditions found in the exposures just 

 north of Forth Henry and within the limits of the Port Henry 

 sheet of the United States Geological Survey map. This area has 

 been described by Professor Kemp, whose report^ contains a 

 description of the limestones here depicted, as is later noted. 



For students of Adirondack geology the vicinity of Port Henry 

 holds much that is of interest; no other part of the mountains 

 probably in so brief a compass has such variety of rock forma- 

 tions, illustrative of the principal types as well as of their general 

 relationships and structural features. The Grenville strata include 

 limestone, schists and gneisses in extensive development; while of 

 the igneous rocks occur granite, syenite, gabbro and trap, and at 

 no remote distance also anorthosite in typical exposures. The 

 early Paleozoic beds which once covered the crystallines are pre- 

 served in small downfaulted patches at the lakeside, ranging in 

 horizon from the Potsdam to the Trenton inclusive. At Mine- 

 ville, 6 miles from Port Henry, are the largest of the Adirondack 

 magnetite mines. 



The Grenville limestone shown in the photographs forms only 

 a small part of the exposed area and is divided into numerous 

 bands interbedded with quartzose, rusty gneisses and schists. 

 According to Professor Kemp's map, the Grenville strata altogether 

 cover an irregular area that reaches some 2 or 3 miles to the north 



'N. Y. State Mus. Bui. 138. 1910. 



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