Reviews — LyelVs Student's Elements of Geology. 181 



parallels of latitude, where we should, from astronomical causes, ex- 

 pect a uniform climate, and points out that " the climates of South 

 Georgia a;nd Tierra del Fuego, in the same latitude as well as in the 

 same hemisphere, are at present so different that the former might be 

 supposed to belong to a glacial period, while the latter, by its flowers 

 and humming-birds in the winter, and the genera of marine mollusca 

 in the adjoining sea, might indicate to the traveller as well as to some 

 future geologist, such a temperature as has been spoken of as per- 

 petual spring. This contrast is due to geographical causes, which if 

 reversed, so that Tierra del Fuego became the oceanic island, would 

 reverse the climate also." (p. 283.) Another striking example is given, 

 p. 242, where we read that in Sandwich Island, in a latitude very 

 nearly corresponding to the north of Scotland, during the hottest 

 time of the year the whole country, from the top of the mountains to 

 the brink of the sea-cliffs, was covered with snow many fathoms thick. 

 So small is the difference of temperature produced directly by the 

 astronomical causes, and so large the effects to be referred to geo- 

 graphical arrangements, even when not aided by astronomical 

 combinations, or perhaps in spite of them. Mr. Croll has, however, 

 suggested some more indirect causes of excessive cold under 

 special circumstances of maximum excentricity, etc. He considers 

 that all the moisture falling as snow, the summer sun, obscured by 

 fogs, would be unable to remove it, and so there would be a storing 

 up of polar ice from year to year to such an extent as even to derange 

 the earth's centre of gravity, to say nothing of its withdrawing so 

 much water from the ocean. 



In reply to this. Sir Charles points out that without abnormal 

 geographical conditions there could be no increase of cold from year 

 to year ; for instance, whenever a deep ocean prevailed at both poles 

 there could be no storing up of ice ; that there was no proof that the 

 increasing heat of summer would not be sufficient to absorb all the 

 aqueous vapour, p; 280 ; that when dry winds blow, snow wastes 

 away like camphor, without melting, p. 289. But space will not 

 allow us to dwell on these interesting questions any longer, and we 

 will only refer our readers to chapter xiii. 



Our author also investigates the origin and distribution of ocean 

 currents and the temperature of the deep sea. This is also very 

 important in its bearing upon ancient climatal conditions : for if we 

 find Arctic cold prevailing at the bottom of Tropical oceans, that 

 must teach us to be cautious in inferring too hastily from the pre- 

 sence of certain forms of life which are now found only in water of 

 a higher or lower temperature, that the general climatal conditions 

 must have been very different from what we now find them in tlie 

 same area. How far a fuller knowledge of the operations of ocean 

 currents may remove some of the difficulties of explaining the occur- 

 rence of tropical plants within the Arctic circle or the dwarfed forms 

 of life of many ancient formations, we must wait to see. 



II. We learn from the preface to the Student's Elements of Geology, 

 that it is founded upon the author's earlier work, the Elements of 

 Geology, which he has abridged by omitting many points of 



