260 TIME CONSIDEBATIONS 



J. G. Gooclchild, certain English limestones waste away, super- 

 ficially, at the rate of one inch in 300 years.^ 



(6) Time Limit of Decay. — "We are sometimes enabled to 

 put a time limit on the beginnings of decomposition such as 

 shall enable us to gain at least a geological measure of the 

 rapidity of the process. This is the ease with the disintegrated 

 granite of the District of Columbia described on p. 185. The 

 residual material is here now overlaid by clastic deposits of such 

 a nature as to force the conclusion that they were laid down by 

 water under such conditions as would have thoroughly eroded 

 away all underlying pre-existing decomposed material. It is 

 therefore inferred that this decomposition has taken place since 

 the clastic material was deposited, or, since these are of Creta- 

 ceous age, that it has taken place since the close of Cretaceous 

 times. In the same way, since glaeiation must have carried 

 away the pre-existing disintegrated matter from the dike of 

 diabase at Medford, leaving the surface smooth and hard, so 

 here it is inferred that the decomposition is post-glacial. It is 

 but rarely that the rate of decomposition of any rock has been 

 sufficiently rapid since the beginning of human history, to be 

 of geological significance, though weathered surfaces in old 

 quarries, or the walls of old buildings offer abundant illustration 

 of what we might expect, could observation be extended over 

 whole geological periods instead of but a few years. It should 

 be remembered, however, that, in the latter ease, the conditions 

 are quite different from those existing in nature, and the rate of 

 weathering may be accelerated or retarded, as the ease may be. 



Stone implements, made by prehistoric man, as now found 

 in graves, or dug from the soil, sometimes show incipient signs 

 of decomposition, as indicated, when broken across, by a change 

 in color and texture from without inward. Flint arrow and 

 spear-heads from prehistoric caves or mounds in Europe, 

 England, or America, often present on the outer surface a thin 

 crust or patine of a gray or white color extending inward, it 

 may be, for the distance of two or more millimeters. A grooved 

 stone axe of diorite found in eastern Massachusetts and now in 

 the collections of the National Museum at Washington,^ shows 

 concentric exfoliation extending inward to a depth of from 



^Geological Magazine, 1890, p. 463. 



® Specimen No. 172,794, Arcliseological Series. 



