130 NATURAL SCIENCE. Feb., 



origin to the fall of temperature, of which we have evidence in the 

 widespread glacial deposits of India, Australia, and Africa. Dubois 

 accepts this view, but does not attempt to discuss the probability of 

 this southern flora being the product of a cold climate. It is difficult 

 to recognise in the plants of this period any characters suggestive of 

 arctic or arctic-alpine conditions. A displacement of the poles is 

 given up as untenable. " Should astronomers and physicists raise 

 no objections to a considerable displacement of the poles, the facts 

 observed with regard to arctic vegetation of the Tertiary period still 

 offer sufficient arguments against such a supposition." 



To discuss the numerous debatable points touched on in this 

 interesting essay would take us far beyond the limits of the space at 

 our disposal. It is satisfactory to find that no very considerable reduc- 

 tion of temperature is demanded for the Glacial period. The author 

 quotes, and apparently accepts, the opinion of Penck : " A general 

 reduction of temperature of 4°-5° C. — and for a climate damper than 

 the present, of even less — suffices to account for the whole Glacial 

 phenomenon." Assuming suitable geographical conditions, we need 

 not have recourse to such a serious lowering of temperature as is 

 frequently deemed necessary. In the second part, the causes of 

 climatal changes are dealt with at some length, and it is urged that 

 we must look to changes of the solar heat for the cause of the 

 geological changes of climate. An attempt is made to examine the 

 sun's history in detail. " The sun is a star like the thousands which 

 appear at night as luminous specks in the celestial vault, even of 

 relatively small dimensions, and differing from the others merely by 

 its closer proximity to the earth. The substances of which it consists 

 are entirely the same as those of which many other stars are 

 composed." The past history of the sun, its gradual change from a 

 star of the first class to a star of the second class, is compared with the 

 history of geological climates. With the sun in the condition of a 

 white star, says Dubois, organic evolution may have progressed 

 faster than in the Tertiary period, when the light and heat of the sun 

 were already diminishing. This part of the essay contains much that 

 is highly suggestive, and is well worthy of attention by those qualified 

 to estimate the value of the evidence and the author's deductions. 



On page u we find the not unusual mistake of applying the 

 common name of mares' tails to the Equisetums, and on page 16 

 Salisburia is written Salisburea. The statement that Dicotyledons^make 

 their first appearance in Jurassic times is not supported by trust- 

 worthy evidence ; we have no traces of these plants prior to Lower 

 Cretaceous times. The essay may be recommended both as a 

 careful compilation of previously-recorded facts, and as an original 

 contribution towards the causes of climatic changes during the past 

 history of the earth. 



A Physical Mantell. 



Open-Air Studies: An Introduction to Geology Out-of-doors. By Professor 



Grenville A. J. Cole, M.R.I. A., F.G.S. 8vo. Pp. xii., 322, with 12 plates 



and 33 other illustrations. London: Charles Griffin & Co., 1895. Price 



8s. 6d. 



In spite of the multiplicity of text-books of geology, there has always 



been room for one which would do for the physical branch of the 



science what Mantell did for palaeontology. A book was wanted 



that would take the student out-of-doors, and there teach him to 



observe and reason on the problems of physical geology, just as 



