PLANT-BEARING SERIES OF INDIA. 533 



to explain its phenomena have not snch presumption of adequacy 

 as to exclude the probability that the principal cause was one of 

 general incidence and is yet to seek. Whether, as some astronomers 

 have speculated, our sun is a variable star of long period, or whether 

 some other cosmical cause as yet undetected has been operative in 

 producing these great changes of climate, is rather a question for 

 astronomers than geologists ; but whatever the cause, to my mind, 

 the balance of probability is in favour of these great oscillations of 

 climate being general and not local merely. 



I am not unmindful of Professor Tyndall's objection, illustrated 

 by the case of the still, viz. that to increase our products of con- 

 densation we must increase the fire under our boiler. This is of 

 course true ; but it does not seem to me that the assumption of 

 wide-spread glaciation is inconsistent with a great reduction in the 

 total amount of condensation in the unit of time. "We can spare all 

 the tropical and subtropical rains, provided that we have the residue 

 that now falls, in part as snow, in temperate and Arctic regions, and 

 a decrease of liquefaction such as would allow of a great depression 

 of the snow-line. If the conditions of Spitzbergen were transferred 

 to the British isles, and those of the Alps to the Himalaya, the total 

 precipitation per annum in each of these regions would be greatly 

 reduced. In Arctic regions, during the period of maximum cold, 

 the precipitation might perhaps be infinitesimal ; and during such 

 time the glaciation of the rocks might then be altogether suspended ; 

 but supposing the change of climate to progress slowly, every zone 

 of the earth, down to the tropical limit of the ice, would be succes- 

 sively glaciated — first during the period of increasing cold, and 

 again during its decrease ; and this is all that the circumstances of 

 the case demand. 



If the probability be admitted that the reduction of temperature 

 evidenced by the glaciation of Postpliocene times was general, we 

 may by analogy argue that such was also probably the case in early 

 Permian times, in which case the physical evidence of glaciation 

 becomes of at least equal value with palaeontological evidence in 

 questions of correlation, perhaps even of greater value when it is a 

 question of correlating formations in very distant regions ; and since 

 the palseontological and stratigraphical evidence warrants the con- 

 clusion that the Ecca conglomerates, the Talchir boulder-bed, and 

 the Permian breccias of England are not very far distant in time, I 

 think we may provisionally refer them with some probability to the 

 same glacial period. 



The comparatively small thickness of the Talchir boulder-bed in 

 latitudes 17° to 23°, as compared with the great thickness of the 

 Ecca conglomerate in latitudes 30° to 32°, would perhaps imply a 

 shorter duration of glacial conditions in the former ; but mere thick- 

 ness is, of course, no safe criterion of duration. Moreover, as sug- 

 gested by my brother, ground-ice in the winter time would be quite 

 competent to produce all the effects to be accounted for in the Indian 

 area, though in Natal Dr. Sutherland appears to consider that the 

 evidence is in favour of a general glaciation of the country. 



