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Such deep-seated rocks, if below the level of no lateral stress, 

 are in the zone of great vertical compressive stress and circum- 

 ferential tension. They would, therefore, be shortened verti- 

 cally. If under the stress of gravity movement goes far enough, 

 this would develop a cleavage parallel to the surface. Such 

 cleavage in sedimentary rocks would be parallel cleavage and 

 would emphasize the bedded structure originally formed. Just 

 below the level of no lateral stress it is probable that the circum- 

 ferential dilation would be slight, but would increase with 

 depth. Whatever its amount, it is a real cause so far as it goes. 



It is not asserted that rocks in which cleavage may thus 

 develop reach the surface by subsequent denudation, but per- 

 fectly crystalline schistose rocks in which the cleavage corre- 

 sponds exactly with the bedding, and which are but gently folded, 

 suggests that such may have been the conditions under which 

 the structure formed, and if the estimates given for the depth of 

 the level of no lateral stress, from two to eight miles, ^ are correct, it 

 is certain that rocks which have been below this level in some 

 regions have subsequently reached the surface by denudation. 

 It is generally believed that the Laurentian and Adirondack 

 areas are regions of profound erosion, and here are found excellent 

 illustrations of gently folded cleavable schists, the structures of 

 which apparently correspond with original bedding. The above 

 explanations may be applicable to these regions. This method 

 of the development of cleavage parallel to the surface of the 

 earth below the level of no lateral stress is also applicable to 

 igneous rocks. 



Laccolitic or batholitic intrusives might promote this process 

 by giving great pressure parallel to the bedding and by heating 

 percolating waters, thus rendering them more active. In the 

 Adirondacks the cleavage of the schists and the periphery of the 

 batholite of gabbro have in some places a parallel arrangement, 

 and the intrusion of the igneous rock has probably been one of 

 the causes of the metamorphism and the parallel relations 



' Origin of Mountain Ranges, by Joseph Le Conte, Jour, of Geol., Vol. I, 1893, 

 pp. 566-568. James D. Dana, Manual of Geology, 4th ed., 1895, pp. 384, 385. 



