324 Notices respecting New Books. 



Since Mr. Croll first propounded his theory of secular changes 

 of climate, a great amount of attention has been given to the 

 arrangement and nature of superficial geological deposits ; many- 

 old facts have been read in the new light shed by additional observa- 

 tion ; and gradually much that was obscure, involved, and perplex- 

 ing has come to take its fit and proper place. These observations 

 have not in any case been made with the view of fitting the facts 

 into Mr. Croll's theory, but were in many cases simply put down 

 by altogether unprejudiced minds as so many isolated facts, which 

 might hereafter be pieced together. Thus by degrees there have 

 accumulated a great variety of observations, all tending to prove 

 that the glacial epoch was as peculiarly characterized by periods of 

 abnormal heat as it was by cycles of rigorous cold. The inevitable 

 inference was at first very staggering to geologists, and they were 

 naturally slow to admit this last and crowning mystery into the 

 long train of perplexities which beset the reading of these baffling 

 records. The discovery of numerous proofs of these warm inter- 

 glacial periods has been one of the best testimonies to the sound- 

 ness of the theory of secular changes, as it is the only theory which 

 not only permits, but requires the occurrence of such oscillations 

 of temperature. Mr. Croll has collected a formidable body of 

 facts proving the interposition of such warm periods ; and he has 

 ingeniously and successfully accounted for the rarity and obscurity 

 of the evidences of interglacial heat as compared with glacial cold. 

 "With the light so obtained he casts his eye backward over the 

 whole geological record, and finds probable evidences of numerous 

 glacial periods in the earth's history, one of the most important 

 and interesting of which forms the Carboniferous period. But 

 this is an inquiry which is yet iu its infancy, although it is not 

 difticult to suppose that results of the highest value may yet arise 

 out of its prosecution. 



Had Mr. Croll written his book entirely cU novo, it is more than 

 probable he would have succeeded in arranging his matter and iu 

 marshalling his facts and arguments in a more systematic and 

 effective form. As it is, his work resembles more a storehouse 

 than a museum : matter of great weight and value is packed up in 

 its chapters ; but these chapters have been in some cases fitted 

 together more with a view to convenience than to their natural 

 sequence and interdependence. This want of continuity would de- 

 tract greatly from the value of the book with popular or holiday 

 readers ; but in no sense does Mr. Croll address such an audience. 

 His work is eminently one for study : it is not simply to be read, 

 but dug into ; and while it will always hold its place among the 

 standard records of science, it will for many a day to come be an 

 indispensable work of reference to all cultivators of the more re- 

 condite problems of physical geography and geology. 



