528 A TREATISE ON METAMORPHISM. 



frequently simulate to a remarkable degree bowlders produced by water or 

 ice erosion. a 



On the Piedmont Plateau, at many places wliere there are good 

 sections exposed, as in railroad cuts, one may see hundreds of these roundish 

 bowlder-like masses in the midst of soft disintegrated material. Erosion 

 carries away the soft material faster than the bowlder-like masses. The 

 latter therefore accumulate at the surface, and finally produce a surface 

 which presents the appearance of a "Felsenmeer." Such a surface has 

 strewn over it innumerable bowlder-like masses, small and large, from 

 those less than a foot to those many feet in diameter. Some of the largest 

 masses are dome-like and of such magnitude that the name "mountain" has 

 been applied to them; for instance, Dunn Mountain, in North Carolina. 

 These masses, small and great, are at the surface simply as a consequence 

 of fracturing, differential weathering, and differential erosion. 



Bowlders of disintegration and mountains of disintegration have been 

 described by Branner 6 as also occurring on a most extensive scale in Brazil. 

 They are beautifully illustrated by the granitic mountains of southern 

 California and other arid regions of the United States. 



The innumerable bowlders in the till of the glacial deposits of America 

 and Europe are believed by some to be largely bowlders of disintegration 

 which were produced before the advance of the ice sheets. This explana- 

 tion premises that before the time of the continental ice sheet the glaciated 

 regions were weathered in a manner somewhat similar to that of the 

 Piedmont Plateau. 



In minor details the nature of the minerals present controls the 

 character of the surface. In general, any surface long exposed to weathering 

 is roughened, owing to difference in the solubility of the minerals, or to 

 imperfect cementation, or to these and other causes combined. The 

 minerals which protrude are usually those which are relatively insoluble. 

 The order of solubility, and therefore of disappearance, is that given on 

 pages 518-521. Very numerous illustrations might be given of the rough- 

 ening of the surface of rocks as a result of the A^ariable rate of solution of 

 the constituents. One of the best illustrations is furnished by the nepheline 



"Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. XJ. S. Geol. 

 Survey, vol. 13, 1888, pp. 68-72. 



^Branner, J. C, Decomposition of rocks in Brazil: Bull. Geol. Soc. America, vol. 7, 1896, pp. 

 269-280. 



