1896.] On the Recurrence of Ice Ages. 115 



Moreover the chief arguments formerly urged in favour of the 

 popular astronomical theory have been found to have been based 

 upon error, and, though some of its supporters still cling to it for 

 auld lang syne, they are waiting till some new facts may turn up 

 in support of it. It seems therefore opportune to urge the terres- 

 trial theory again. 



There seems to be an attempt to retreat behind the well-known 

 fact that ice makes ice, or the cumulative effect of refrigeration, 

 especially beyond a certain point. But it seems to be forgotten 

 that this applies equally to terrestrial causes, and that if a moun- 

 tain range were upheaved, say, a thousand feet above what would 

 be the estimated snow line for that locality, the existence of that 

 snow and ice would so lower the temperature that practically the 

 snow line would creep down far below the altitude estimated from 

 considerations of latitude, longitude and level only. 



Further, I must point out in passing that the demonstration 

 offered by Professor David does not provide a bill of indemnity for 

 those who have been claiming credit for the discovery of glacial 

 beds and have offered in proof only slickensided pebbles and 

 similar unsatisfactory evidence. At the Geological Society when 

 Professor David exhibited, among others, these specimens which 

 he has kindly presented to the Woodwardian Museum, a stone 

 was exhibited from the Indian boulder beds. This I had no 

 hesitation in pronouncing to be merely a slickensided pebble, 

 a view in which Professor David entirely concurred. Dr Heim 

 has written to me in answer to enquiries I had addressed to him 

 in regard to the stone I had noticed in the Museum at Zurich and 

 said to have been found in situ in the " crystalline boulder beds," 

 or "palaeozoic glacial bed" in the Salt range of Sandschal in India. 

 "The striations," he says, "are different on different distinctly 

 cornered sides. That is a phenomenon of quite another order 

 than glacial striation." 



As I have before explained, the character which is most 

 common in pebbles which have been subjected to crushing action 

 in a conglomerate, and most clearly distinguishes them from 

 glacially striated rocks is the manner in which they often appear 

 to be pinched out and scored in the same direction on both sides 

 up to the edge ; so that, whereas the same set of glacial striae 

 seldom run far over the margin on the surfaces of any glacial 

 boulder, and flat sides are the rule, in crushed conglomerates, on 

 the other hand, the grooves follow the curvature of the stone, and 

 the edge is sometimes crushed up as if the pebble had been nipped 

 out. 



One of these Australian specimens shows furrows due to the 

 weathering of the rock along weaker divisional planes, but these 

 cannot be confounded with the true glacial stria?. We may, I 



8—2 



