210 R. PUMPELLY — SECULAR ROCK-DISINTEGRATIOK. 



begins with disintegration and ends with the reduction of the rock to the 

 most insoluble products, such as quartz, clay and ferric oxide. The depth 

 of this decay, other things being equal, is determined by lapse of time, by 

 the permeability of the rock, and by the solubility of its constituents, rather 

 than by its hardness. In tropical countries, and in our southern mountains, 

 this depth is to-day measurable in places by hundreds of feet. I showed that 

 when, by change of climate, the protecting vegetation was destroyed and the 

 disintegrated region became arid, this great decayed mantle became the 

 prey of the winds, and thus furnished the material for the wind-blown loess. 

 Where, on the other hand, such a region became the seat of a continental 

 glacier, this decayed mantle supplied, in its finer material and in its cores 

 of semi-disintegrated blocks, the source of the greater part of the glacial 

 debris. 



Since the limit of the decay in depth is due to the character of the rock 

 areas, the removal of the mantle by wind or ice would leave a topography 

 different from that formed by stream erosion, and one in which rock basins 

 would be frequent. 



Finally, the rapidity with which this material, after accumulating by wind 

 or ice, is removable by erosion, or by progressive ocean breaching, rendering 

 turbid the waters of formerly clear parts of the sea, suggests the cause for 

 the extinction of life and change of coast faunas. 



The views expressed in the paper referred to were accepted in their en- 

 tirety by von Richthofen * and, as bearing on glacial debris, rock basins 

 and the topography of Scandinavia, by Nathorst.f 



Evidence of secular Disintegration in ancient Rocks. 



Derivation of Cambrian basal Conglomerates. — Our work in the Green 

 mountains, and recent studies of the mountains of western North Carolina, 

 have given me proof that the recognition of the importance of secular disin- 

 tegration is essential to a proper interpretation of some of the most difficult 

 points in the study of the crystalline schists. Throughout the Green mountains 

 and the Appalachians, the Cambrian conglomerates and quartzites, resting on 

 an older crystalline complex, contain large quantities of detrital feldspar in 

 fragments or pebbles, up to three-quarters of an inch and more in diameter, 

 together with grains and pebbles of blue quartz, all clearly derived from the 

 destruction of the older granitic rocks. These feldspars are the same as 

 those in the older rocks, and show their own detrital character. They often 

 show partial kaolinization around or adjoining an unaltered nucleus. And 

 in some cases these fragments, as my assistant. Dr. Wolff, finds, have been 



* China, Vol, ft, 1882, p. 758. 



fPumpelly's teori om hetgdelsen af herzartemas sekalara forwittring for uppkomsten af Sjoar 

 m. m."— Geol. Foreniogens i Stockholm Forhandl, 1879, No. 52, Bd. IV, No. 10. 



