Reviews — Lyell's Princi2)les of Geology. 179 



For forty years has the progress of the science been watched, and 

 the i-esults of varied research carefully examined by one who can 

 say in almost all branches of Geological inquiry, Quorum po/is magna 

 fui. Hence, while the general reader turns to Sir Charles Lyell's 

 works for the most trustworthy account of what has been discovered 

 by geology, and the inferences which may be drawn from those 

 discoveries, the geologist turns to each new edition to see what is 

 his judgment upon the more speculative questions which have been 

 under examination since the appearance of his last edition. 



Most of the readers of the Geolooioal Magazine are familiar 

 with and accept the Principles of Geology. We will therefore 

 confine ourselves chiefly to some of the points which have been 

 recently under discussion. 



Sir Charles Lyell has made no change in the great Principles 

 upon which he bases all his inferences. When we see phenomena 

 precisely analogous to effects of which we know the cause, we must 

 explain those phenomena by referring them to similar agencies to 

 those which we now see producing precisely similar results — 



JSTec deus iutersit nisi dignus vindice nodus. 

 But at the same time he shows us that it is part of Nature's plan by 

 the accumulation of small effects, to produce great results. 



Our author excludes from his inquiry all speculation as to what may 

 have been the origin of the earth, and, holding that " Geology differs 

 as widely from Cosmogony as speculations concerning the mode of the 

 first creation of man differ from History" (p. 5), considers only the 

 mode of formation of that portion which comes under our observa- 

 tion, and the conditions which must have prevailed at the several 

 periods of its history. But while he passes by such questions as the 

 nebular hypothesis, and the original fiuid condition of our planet, as 

 foreign to the scope of his work, there are astronomical questions 

 of the greatest importance to geology which he goes into very fully. 



It is a well-ascertained fact that there have been great vicissitudes- 

 of climate from very early periods. Here we have the fern and the 

 vine, where boreal cold once prevailed; and there conditions are 

 reversed, and we fiad that magnolias once bloomed within the Arctic 

 regions.^ 



We need not pause long over the view which would account for 

 observed changes of climate by the theory that the earth has been 

 cooling down from an original high temperature. Sir Charles has 

 expressly combated it (p. 296); and appealing to some calculations of 

 Laplace on the length of the day, points out that " there are no 

 positive proofs of a secular decrease of internal heat accompanied by 

 contraction." In chap. vii. also lie has given a clear refutation of 

 the doctrine of the supposed former intensity of the igneous forces, 

 which to a certain extent involves the same question. This does not 

 imply that our author holds there has been no such gradual cooling 

 down of our planet, but only that if there has, it must have gone on 

 to such an extent before our history begins that we have no evidence 

 of any perceptible difference within the period of which the rocks 

 ' Student's Elements, p. 215. 



