TIME LIMIT OF DECAY 



261 



three to six millimetres, and comparable to that on the diabase 

 boulder figured on PL 22. It is of course possible that the axe 

 was made from a boulder, itself not quite fresh, but this seems 

 scarcely probable, and the inference is fair that both the patine 

 and the exfoliation are due wholly to weathering subsequent to 

 the manufacture of the implements on which they occur. 



Mills^ regards the extreme condition of decomposition exist- 

 ing in the Archaean rocks of Brazil as having taken place prior 

 to the deposition of the loess, that is, in the long interval between 

 the elevation of the Archsean rocks and the beginning of Qua- 

 ternary times. Inasmuch, however, as the Quaternary gravels 

 and loess are all readily permeable by water and not of a nature 

 to be themselves readily affected, it would seem possible that 

 at least a portion of the decomposition might have been brought 

 about since their deposition and, indeed, be still in progress. 



The writer is informed by Mr. W. Lindgren that the granitic 

 diorites of the Sierra Nevadas of California, which are of late 



Fig. 22. — ^Plint implement showiiag weathered surface. 



Jurassic or early Cretaceous age, are often decomposed and dis- 

 integrated to a maximum depth of 200 feet, the extreme upper, 

 ^American Geologist, June, 1889, p. 345. 



