NA TURE 



385 



THURSDAY, AUGUST 23, ll 



BRITISH PETROGRAPHY. 

 British Petrography : with Special Reference to tin: 

 Igneous Rocks. By J. J. Harris Teall, M.A., F.G.S. 

 With Forty-seven Plates. (London : Dulau and Co., 

 1888.) 



THIS handsome volume, with its beautifully chromo- 

 lithographed plates, supplies a want that has long 

 been felt in English scientific literature. It was scarcely 

 fitting that in this country, where the application of the 

 microscope to the study of thin sections of rock was first 

 suggested and practically carried out, there should exist 

 no comprehensive work dealing with the chief varieties 

 of our native rocks, as illustrated by their microscopic 

 characters. 



In its general appearance, plan, and scope, this volume 

 reminds one so closely of the " Mineralogie Micro- 

 graphique : Introduction a l'Etude des Roches Eruptives 

 Franchises," of MM. Fouque' and Michel Le"vy, that it is 

 scarcely possible to avoid a comparison between the two 

 works. Artistically, the forty-seven plates of the English 

 treatise may perhaps even claim superiority over the 

 fifty-five plates in the French work ; though in the exact 

 presentation of minute but characteristic details, and in 

 the accuracy of tints employed, the palm must in some 

 cases be awarded to the latter. There are some plates in 

 the volume before us, however, in which truthful delinea- 

 tion of details has been so admirably combined with a 

 general beauty of effect as practically to leave nothing to 

 be desired in work of this class. 



Like his French predecessors, the author of this volume 

 has found it desirable to go outside of the country 

 illustrated for a few of his types of igneous rock. A 

 striking testimony, however, to the variety as well as the 

 beauty of our native rocks is found in the circumstance 

 that it has been possible to present so complete a 

 selection of the chief types of igneous materials without 

 going beyond the limits of the British Isles except in two 

 instances, — those, namely, of the Lherzolite of the Ariege, 

 and of the Lencitic rock of the Eifel. Nor are the 

 varieties of British igneous rocks by any means exhausted 

 in the illustrations of the work before us. The rhyolites. 

 which perhaps are less adequately represented than some 

 other groups, might have had their more crystalline 

 varieties (Nevadites) well illustrated by the beautiful 

 rocks of Tardree, Co. Antrim, while examples of trachyte 

 of graphic-granite, and of various types of granulites and 

 " trap-granulites," might have been easily obtained from 

 Scotland. On the whole, however, we think the author 

 has shown excellent judgment in his selection of types, 

 and he is to be heartily congratulated upon his success in, 

 securing accurate drawings, and exact reproductions of 

 those drawings by the process of chromolithography — 

 results which we are assured could not have been 

 attained without much labour and extreme care. 



Although the book is one which is especially note- 

 worthy for the beauty of its illustrations, it would be a 

 mistake to suppose that it belongs to that class of works 

 in which everything else is sacrificed to showy plates, and 

 Vol. xxxv;.[.— No. 9S2. 



scientific accuracy is regarded as merely a secondary 

 object. On the contrary, the author has clearly devoted 

 great pains to the perfecting of his text, which constitutes 

 in itself an excellent introduction to the study of petro- 

 graphy. Some of the rocks chosen for illustration 

 have already been described by other authors, and in 

 these cases Mr. Teall, while doing full justice to the 

 labours of his predecessors and contemporaries, has not 

 unfrequently been able to extend, supplement, or correct 

 their results by the light of more recent researches ; in the 

 case of rocks which have not been previously described, 

 the author has himself investigated their chemical and 

 microscopical characters, in some instances in a very 

 complete and exhaustive manner. In all cases he has 

 earned the gratitude of students by the copiousness of his 

 references to the ever-growing mass of literature which 

 deals with the question of the minute structure of minerals 

 and rocks. 



While MM. Fouque and Levy have devoted the text of 

 their work to a systematic description of the various 

 species of rock-forming minerals, and especially of those 

 characters which enable us to recognize them when seen 

 in thin sections under the microscope, the author has 

 aimed rather at describing the rocks themselves, inci- 

 dentally discussing the characters of each species of 

 mineral as it presents itself in the different groups of 

 rocks. This plan, while attended with certain advantages, 

 may perhaps be objected to on the ground that it is only 

 possible to gather the whole of the conclusions of the 

 author upon any particular mineral after consulting dif- 

 ferent and widely-separated portions of the book. This 

 is rendered more easy, however, by the very full index 

 which is supplied. 



The work, we are informed in the preface, was com- 

 menced as a serial publication, and to this cause probably 

 must be ascribed its most serious defect as a means of 

 instruction : this is the absence of references and cross- 

 references between the text and the atlas of plates ; 

 these, indeed, constituting two practically independent 

 works. Had all the plates been before the author during 

 the time that he was preparing the text, he would 

 frequently have been able to illustrate his remarks upon 

 the minerals and structures in the rocks he is describing 

 by references to his own admirable drawings. To the same 

 cause, too, we must ascribe the only other serious blemish 

 we have detected in the book — a rather large proportion 

 of misprints, which, though usually obvious enough to the 

 initiated, may occasion considerable embarrassment to 

 the student. 



However much the beginner, taking up this attractive 

 volume, may be delighted with the mode of study of 

 which it aims at giving an exposition, he will scarcely 

 be led into the fatal error of supposing that everything 

 necessary to a person seeking to employ the method is a 

 microscope and some rock-sections. The author makes 

 it perfectly clear that unless the student is prepared to go 

 through a certain amount of preliminary training, the 

 microscopic examination of a rock is more likely to lead 

 to error rather than to truth. So much knowledge of 

 crystallography as will enable the observer to appreciate 

 the position of any section with respect to the axes of the 

 crystal, and such an acquaintance with the principles of 

 physical optics as will suffice to guide him in interpreting 



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