LOWERING OF IRON MOUNTAIN BY DISINTEGRATION. 221 



much higher than when the up-stream part of the bed obtained its present 

 slope. The greater height and steeper declivity would allow the blocks to 

 roll further and more rapidly, while, as the height of the hill and the slope 

 of the valley diminished, the ore debris was finally deposited nearer its 

 source. The valley bottom is exposed in the mine and shows all the mark- 

 ings of torrent erosion. 



Any attempt to state the amount of diminution in height that Iron 

 mountain has suffered by disintegration, can, of course, be only a very rough 

 estimate. But, considering the relation between the size of the known ore 

 bodies and the volume of ore debris, so far as known, the lowering of the 

 height of the mountain cannot well be placed at less than 150 feet since the 

 Silurian transgression, and probably as large an amount before that time, 

 or a minimum of 300 feet. 



At Pilot knob, we have, apparently, evidence not only of a similar pre- 

 Silurian mantle of ore debris around the base, but also of a much older dis- 

 integration in Algonkian time, in the occurrence of a conglomerate of por- 

 phyry and quartz pebbles with a cement of specular ore, forming the top of 

 the mountain. 



Formation of Transition Beds. — Professor Van Hise, knowing that I was at 

 work on this subject, and being himself engaged in reviewing the literature 

 of the pre-Cambrian rocks of America, kindly noted and called my attention 

 to the following instances illustrative of the theory here advanced : 



Newton * described the Potsdam sandstone exposed in the canon of French 

 creek, in the Black hills, as lying horizontally on — 



" Coarse, red, feldspathic granite, at top very much decomposed and changed into 

 a soft clay, almost like a fiuccan." 



Professor Van Hise adds : 



" One can readily imagine that, folded and metamorphosed, there would be a 

 gradual transition from the thoroughly crystalline rock to that which was originally 

 a sediment." 



Peale,t describing a section on the South Platte in Colorado, says : 



" Resting immediately upon the granite we have a very coarse sandstone ^ -s^- *. 

 Close to the granite the sandstone is coarsest and contains pieces of unchanged granite. 

 In other places the sandstone appears to pass by gradation into granite." 



Stevenson J describes at several localities on the Gunnison and Grand 

 rivers and elsewhere a peculiar regular laminated gneiss, dark brown or 

 black, and resembling a micaceous sandstone. He adds : 



* Geology of the Black Hills, 1880, p. 90. 



t7th Ann. Rep. Geol. and Geogr. Surv., 1874, p. 194. 



JGeoI. Surveys West of the 100th Mer., vol. Ill, 1879, pp. 344, 345. 



