EARLY CRUSHING OF DISINTEGRATED GRANITE. 219 



wavy cleavage, are penetrated by the mica which, in blotches, marks the 

 foliation of the rock. This rock presents, in the wavy cleavage of its feld- 

 spar, in the granulation of the feldspar and quartz, and in the foliation ap- 

 parently contemporaneous with the crushing, the characteristics which should 

 result from the action of a folding force upon the lowest zone of disintegra- 

 tion ; the zone in which the integrity of the granite is weakened. 



Of course we are dealing here with a period of much greater age than the 

 Cambrian ; and the conditions of depth aud pressure may have presented 

 very different relations to the rigidity of the granite from those ruling dur- 

 ing the folding of the Cambrian sediments. But, with the evidence of a 

 previous disintegration presented by the detritus in the Adirondack lime- 

 stone mentioned above, I am disposed to ascribe the crushing and foliation 

 of the granite underlying the old limestone to the action of the folding force, 

 probably in pre-Cambrian time, upon a rock which had lost its integrity 

 before the formation of the limestone. 



Accumulation of Ores. — If the reader wishes to see a convincing illustra- 

 tion of a deep-reaching disintegration in pre-Silurian time, in a region which 

 has not been folded, I know of no better instance than that now offered at 

 Iron mountain, Missouri. Before the beginning of mining operations, Iron 

 mountain was a hill covered, to an irregular depth, with blocks of magnetic 

 ore altered to raartite. In earlier geological times, it was a mountain of non- 

 quartziferous porphyry, traversed by several large masses of magnetite, from 

 which veins and veinlets of the same ore ramified in every direction through 

 the porphyry. As the rock decomposed and was washed away, the ore re- 

 mained to form a mantle of residuary fragments. In 1873,* I called atten- 

 tion to this, as an instance of ancient disintegration, and stated that beds of 

 stratified ore-conglomerates occurring at the base of the mountain were prob- 

 ably of Silurian age, and that they indicated a pre-Silurian disintegration of 

 the mountain mass. 



During the present summer I revisited the locality, with Professor Van 

 Hise, and found a startling confirmation of the former statement. 



Professor W. B. Potter, reasoning from the premise of a pre-Silurian dis- 

 integration, instituted a series of borings through the Silurian strata which, 

 in a flat position, surround the base of the mountain. These explorations 

 resulted in the discovery of extensive areas of residuary ore-fragments lying 

 on the pre-Silurian surface, and in the development of mining operations for 

 their recovery. 



On the Avestern flank of the mountain (figure 4) the original surface of 

 decomposed porphyry, grading downward into less altered rock, slopes down- 

 ward at an angle of 28°, under strata of lower Silurian or upper Cambrian 



* Geological Survey of Missouri, Iron Ores and Coal Fields, 1873, part I, p. 12. 



