272 TIME CONSIDEBATIONS 



disintegrated to a depth of from 20 to 30 feet. Belt^ describes 

 dolerite in Nicaragua, as shown by steep cuttings in mines, de- 

 composed to a depth of 200 feet. ^^Next the surface," he says, 

 ^'they were often as soft as alluvial clay, and might be cut with 

 a spade." 



Derby describes'' certain shales in Eio Grande do Sul, Brazil, 

 reduced by decomposition to the condition of reddish, drab, green- 

 ish, black, and umber-colored clays to the depth of 120 metres 

 (394 feet), and W. H. Furlonge has described^ the granite of 

 the Dekaap gold fields, in the Transvaal, South Africa, as de- 

 composed to a depth of 200 feet. Eain erosion has carved out 

 from this decomposed mass deep 'dongas," as they are locally 

 called, which sometimes present more striking and picturesque 

 appearances. 



The apparent depth to which weathering has gone on is 

 often greater among siliceous than calcareous rocks. This is, 

 however, due merely to the facts that (1) the siliceous rocks 

 are composed largely of insoluble materials, and hence leave a 

 proportionately large amount of debris, and (2) that among 

 calcareous rocks the change is mainly chemical and takes place 

 only from the immediate surface. As a result of this, residuary 

 nodules o£ limestone may be found perfectly fresh and unal- 

 tered at a depth of but a few millimetres below the surface, 

 while granites and allied rocks may show signs of disintegra- 

 tion and incipient decay for many inches, or even feet. 



Pumpelly states^ that in the Ozark Mountains of Missouri 

 the secular dissolving away of limestones containing from 2 to 

 9% of insoluble matter has left residual clays from 20 to 120 

 feet in thickness, indicating a removal of not less than 1200 

 vertical feet by solution. According to "Whitney, the dark, 

 reddish brown, residual clays of southern Wisconsin, of an 

 average depth of perhaps 10 feet over the entire area, repre- 

 sent the insoluble accumulations from the decomposition of 

 from 350 to 400 vertical feet of dolomite, limestone and calcareous 

 shale. As a considerable portion of the residue in any area un- 

 dergoing decay is being continually removed through the action 



1 The Naturalist in Nicaragua, p. 86. 



2 Am. Jour, of Science, February, 1884, p. 138. 



3 Trans. Am. Inst, of Mining Engineers, Vol. XVIII, 1890, p. 337. 

 *Am. Jour, of Science, 1879, p. 136. 



