472 Reviews — Sir W. Thomson's Geological Dynamics, 



figures to illustrate the structure and parts of a Trilobite, to render 

 the plates of Silurian forms more intelligible. In referring to the 

 Graptolites we have quite a number of forms of Virgularia, Sertularia, 

 etc., given as recent illustrations. 



In the descriptive remarks on Corals in Part II. we have figures 

 in the text to explain the several parts of a Coral and its mode of 

 reproduction by budding from the calice, the side, the base, or by- 

 fission ; also the distinction between Palaeozoic and Neozoic types 

 of corals are plainly shown by diagrammatic woodcuts. 



Again, under the descriptive remarks on the Crinoids, we find 

 the same useful method of teaching introduced, so that one after- 

 wards readily seizes upon the leading features in a description of 

 any particular species or genus. 



The lithographic plates appear brighter and sharper in Part II., 

 which we take to be a sign that even a distinguished paleeontologist 

 and draughtsman like Mr. Baily goes on improving, like good old 

 port; but we hope to have occasion to enjoy the pleasure of record- 

 ing the issue of fresh parts of Mr. Baily's work more often than we 

 have of tasting " thirty-four." 



YII. — GrEOLOGICAL DYNAMICS. 



By Sir "William Thomson, LL.D., F.E.S. 



[Reprinted from tlie Transactions of the Geological Society of Glasgow, vol. iii., part ii.] 



Part I. — Beply to Professor Huxley's Address to the Geological 

 Society of London, of February 19, 1869. 



IN his Anniversary Address to the Geological Society of London, 

 Professor Huxley directed attention to two sentences in a 

 lecture on " Geological Time," delivered by Sir W. Thomson. They 

 were as follows : — 



''A great reform in geological speculation seems now to have become necessary. . . 



. . . It is quite certain that a great mistake has been made — that British 

 popular geology, at the present time, is in direct opposition to the principles of natural 

 philosophy." 



In the first place he endeavoured to show that, even if geological 

 time were limited to 100,000,000 years (a period to which Sir W. 

 Thomson's calculations tend to show it may be restricted), no great 

 reform in geological speculation would become necessary, and that 

 the limitation may be accepted without a complete revolution in our 

 ideas. Thus : estimating the total thickness of stratified rocks con- 

 taining traces of life to be 100,000 feet. Professor Huxley shows 

 that the whole thickness might have been deposited in 100,000,000 

 years, at the rate of -j-qVo °^ ^ ^°'^^' ^^ ^^J tV ^^ ^^ inch, per annum. 

 In the second place, he examined the arguments upon which Sir 

 W. Thomson's calculations were based, and pointed out that if the 

 data upon which they are founded are loose and uncertain, as he 

 himself admits, no amount of mathematical accuracy can render the 

 deductions drawn from his calculations of any serious import to the 

 geologist. 



