EATE OF WBATHEEING " 271 



actually creep slowly down the slopes, the movement taking 

 place principally in the winter time and being due apparently 

 to the slow settling, or creep, of deep snows. He states it as 

 his opinion that the mountains of the region have suffered more 

 through this form of disintegration than have those of Colorado 

 or the southern Appalachians, but less than those of the Great 

 Basin area. The range of limestone mountains along the Yukon 

 is pictured as presenting a crest of sharp, blade-like crags, flanked 

 by vast slopes of loose, angular stones on either side, the rock 

 being everywhere fresh and undecomposed. but badly shattered 

 and fissured. 



(9) Extent of Weathering. — The depth to which weather- 

 ing has penetrated necessarily varies greatly. In cases where 

 the detrital material is removed nearly or quite as rapidly as 

 formed, it may go on indefinitely, until, it may be, thousands 

 of feet of material have melted away ; where, however, remain- 

 ing in place, decomposition must be gradually retarded until a 

 time comes when it practically ceases. In the region about 

 Washington, District of Columbia, the writer has observed the 

 granitic rock so disintegrated at a depth of 80 feet from the 

 present surface as to be readily removed by pick and shovel. 

 Even greater depths have been noted by writers on the geology 

 of our own Southern states and Central and South America. 

 Spencer states^ that in the region about Atlanta, Georgia, the 

 rocks are '* completely rotted" to a depth of 95 feet, while 

 ^^ incipient decay" may reach to a depth of 300 feet. W. ]B. 

 Potter describes^ the feldspar porphyry of Iron Mountain in 

 Missouri as decomposed to a visible extent as far into the hill as 

 mining operations had been carried, while to depths varying from 

 10 to 80 feet the kaolinization is complete. C. W. Hayes has noted 

 that diorites in the Chattanooga district of Tennessee are often 

 weathered to the condition of incoherent sand for a distance of 

 from 50 to 75 or even 100 feet from the surface,^ while Sterry 

 Hunt, as long ago as 1875, called attention^ to the evident signs 

 of weathering in the rocks pierced by the Hoosac tunnel, in 

 Massachusetts, at a depth of 200 to 300 feet. 



The coarse granite of Pikes Peak, Colorado, is reported as 



^ Geol. Survej^ of Georgia, 1893. 



=* Jour. XT. S. Assoc. Charcoal Iron Workers, Vol. YI, p. 25. 



« Nineteenth Ann. Kep. IT. S. Geol. Survey, 1897-98, Part II, p. 18. 



* Trans. Am. Inst, of Min. Engs., Vol. Ill, 1875. 



