268 Reviews — C. L. GrieshacJi's Central Himalayas. 



sufficiently accentuated. Dr. Croll, on the other hand, has shown, 

 as we have seen, that, according to his views, the astronomical con- 

 ditions may occur without leading to glaciation. Hence, according 

 to the Revised Theory, a larger number of glacial periods in the past 

 is necessary, and hence a larger amount of evidence of such should 

 be expected from the geological record. 



The imperfection of the record doubtless accounts for much, yet it 

 is difficult to believe that glacial periods could have occurred in the 

 past with the frequency demanded by the Revised Astronomical 

 Theory without leaving much more evidence of their occurrence 

 than has yet been forthcoming. 



In conclusion, I have not wished in the foregoing remarks to 

 show that the Astronomical Theory is insufficient to account for 

 glaciation. I have rather endeavoured to call attention to certain 

 calculations which will have to be made, and certain facts which 

 will have to be established before the Revised form can be generally 

 received as a satisfactory explanation. Thus it will have to be 

 shown, by careful calculation and comparison with existing climates, 

 that the reduction of the average winter temperature by the 

 lengthening of that season would be sufficient to produce in our 

 latitudes a snowfall comparable to that of Greenland at the pi'esent 

 day. And granting such a snowfall, it will have to be shown that 

 all the heat of the succeeding summer — the same in amount as now — - 

 would be insufficient to melt it. And in order to calculate the 

 reduction in temperature, due to the lengthening of the winter, a 

 suitable datura line of temperature will have to be fixed upon, after 

 a careful consideration of the numex'ous factoi's involved. 



The supposed confirmation of the Astronomical Theory in the 

 present distribution of temperate plants north and south of the 

 equator, I have endeavoured to show is i-ather a contradiction, since 

 the burning heat of the equatorial summer during glaciation would 

 render the passage of such plants across the equator even more 

 unlikely than it is to-day. 



I have demurred again to the suggestion that Sir R. Ball's dis- 

 covery removes any difficulty from the older Theory of Dr. Croll, 

 and have drawn special attention to the essential diiforences between 

 the two views. I have pointed out that the newer view more 

 urgently demands evidence of former glacial periods in the geological 

 record, and noted the easy way in which Sir R. Ball proposes to 

 meet this demand. 



I. — Geology of the Central Himaliyas. By C. L. Griesbach, 

 C.I.E., Superintendent, Geological Survey of India. Memoirs 

 of the Geological Survey of India, Vol. XXIII. 1891, pp. 232, 

 Pis. 27, Phototypes in the text 31, and 2 Maps. 



BY the term 'Central Himalayas' the author includes that 

 portion of the system of mountain ranges fringing the 

 southern margin of the the Tibetan Highlands, in which the head- 

 waters of the Ganges drainage are situated, and which extend to 



