420 Reviews — Harker's Igneous Rocks. 



by nearly complete skeletons, and several which appeared to have 

 a restricted geographical range have now been found over a much 

 wider area ; but while this progress has been made, numerous 

 questions have arisen as to the changing connexions of certain lands 

 and seas which previously seemed to liave been almost settled. The 

 outlook both of zoology and geology has, therefore, been immensely 

 widened, but the only real contribution to philosophy has been one 

 of generalities. Some of the broad principles to which I have referred 

 are now so clearly established that we can often predict what will 

 be the main result of any given exploration, should it be successful in 

 recovering skeletons. We are no longer bold enough to restore an 

 entirely unknown extinct animal from a single bone or tooth, like 

 the trustful Cuvierian school ; but there are many kinds of bones and 

 teeth of which we can determine the approximate geological age 

 and probable associates, even if we have no exact knowledge of the 

 animals to which they belong. A subject which began by providing 

 material for wonder -books has thus been reduced to a science sufficiently 

 precise to be of fundamental importance both to zoology and to geology ; 

 and its exactitude must necessarily increase with greater and greater 

 rapidity as our systematic researches are more clearly guided by the 

 experience we have already gained. 



I^E"VIE-V\rS. 



I. — The Nattjeal History of Igneous Rocks. By Aifeed Haekee, 

 M.A., F.E..S. 8vo ; pp. xvi, 384, with 112 diagrams and 

 2 plates. Methuen & Co., London. Price 12s. Qd. net. 



Tj"^OE, more than twenty years Dr. Harker has advocated that the 

 J; igneous rocks should generally be treated in reference to 

 geological age and to the part which they have played in the tectonics 

 of the earth's crust. Many papers have been published that consist 

 of descriptions of the microscopic structure and mineral constituents of 

 rocks, with little or no reference to the geological history of the 

 region from which they have been collected. This descriptive side of 

 petrology, or petrography, is but a part of the 'rational petrology' 

 that should be dealt with in relation to historical geology. 



It must be remembered, however, that in many cases it has not been 

 possible to determine the particular age of intrusive rocks, and it is 

 therefore not surprising that petrographical details have often been 

 "relegated to a more or less perfunctory appendix" in various 

 publications. Our knowledge, however, has vastly increased during 

 the past quarter of a century, a fact very clearly shown in the volume 

 before us. 



While recognizing that " cosmogony is at least as much in the 

 province of the geologist as in that of the astronomer", the author is 

 content to refer very briefly to hypotheses relating to the genesis of 

 our planet, and to the possible influence of radium on the temperature 

 gradient, as "the present situation is so unsettled that one who is not 

 directly led to discuss these large questions may legitimately adopt 

 towards them an attitude frankly agnostic". Moreover, "the 



