28 



NA TURE 



[November 14, 1895 



THE CLASSIFICATION OF ROCKS. 

 An Introduction to the Study of Rocks. By L. Fletcher. 

 (British Museum (Natural History) Mineral Depart- 

 ment, 1895.) 



HAVING received from the facile pen of Mr. Fletcher 

 a guide to the , Mineral Gallery of the British 

 Museum, and an introduction to the Collections of Minerals 

 and Meteorites which it contains (works of a very high 

 order from their simple lucidity as well as their thorough- 

 ness and accuracy) the student has looked forward to a 

 companion work on the Rock Collection, from the same 

 writer. This work has now made its appearance, and 

 is no whit behind, its fellows in outward aspect, being 

 printed in clean, clear type, on good white paper, cheap, 

 not too bulky, and, attractive in its general appearance. 



A large part of the work is taken up with an essay 

 on classification, which will certainly tend to impress the 

 student with the care that must be used in seizing on 

 essential characters, the numerous pitfalls to be avoided, 

 and the necessity of employing every instrument of re- 

 search available in the study of rocks. After some pre- 

 liminary paragraphs on the varied points of interest which 

 rocks present ; on the chief characters presented by the two 

 constituents of rocks, minerals and amorphous matter; 

 and on their ordinary modes of origin ; the author ob- 

 serves that rocji-masses vary so much from point to point, 

 that " siiyiilarity, not identity " of characters can alone 

 be aim^d at in a classification. He strives to recognise 

 the existence of '•'' petrical individuals^^ . but, owing to 

 .variation in different parts, to alteration, inclusion, and 

 denudation, fails to reach the, ideal, and is compelled 

 to state that " peirical individuals haye rarely, if ever, 

 existed." We can hardly realise, however, that the 

 inclusion of fossils or pieces of foreign rocks, which seem 

 to trouble the orderly mind of the author, can have any 

 real bearing on nomenclature ; a boy is no less a boy 

 if he happens to have swallowed a button. The out- 

 come of this discussion is that "a rock-name is only 

 required by the mineralogist for the purpose of indicating 

 the kind of rock, not the particular rock-mass itself." 



An admirable account is then given of "lithical 

 characters " observed in hand specimens, both in relation 

 to their individual constituents and to their aggregation 

 or structure, and of the " petrical characters," which are 

 only to be observed on a large scale in the field. A 

 very useful term is here introduced — " merocrystalline," 

 which is to be correlative with holocrystalline ; all petro- 

 'logists will be grateful for this substitute for semi- 

 crystalline and the other unsatisfactory terms that they 

 have been compelled to employ under protest. Taking 

 the characters here specified, the author employs them 

 to construct a tentative classification of a set of typical 

 rocks, by linking into one group those which have several 

 of these important characters in common. The first 

 .scheme attained by this method is in part natural, in 

 .that it brings together those " rocks which are composite 

 in. kind of material, holocrystalline, and without direc- 

 tional lithical characters," . such as granite, syenite, 

 ,diorite, dolerite, and euphotide. But it js also in part 

 artificial, as, for instance, when it brings together gneiss, 

 •lihale, and slate because they possess directional char- 

 acters, and coal, clay, and phonolite because they are 

 NO. 1359, VOL. 53] 



compact though composite. Mr. Fletcher points out 

 that " directionality has been useful, however, in enabling 

 us to bring together the rocks belonging to the several 

 kinds " ; but that things thus wedded are to be so quickly 

 divorced is, we take it, his method of enforcing the 

 necessity for most careful selection of essential characters 

 in classification. The primary essential, when hit upon, 

 turns out to be mode of origin. 



, From this point things go more smoothly, and the 

 rocks fall into a grouping which is, for the most part, 

 natural ; want of complete knowledge on such subjects 

 as the origin of the crystalline schists still, however, 

 leaves us in difficulties in classifying these rocks, and we 

 are compelled to place in an artificial group many which 

 differ widely in their methods of origin. 



In developing the natural grouping finally adopted, 

 the history of the granite and basalt controversies is 

 succinctly told, and a set of useful definitions and de- 

 scriptions of the chief types of rocks is given. The work 

 closes wifti a brief syllabus of these types, which have, 

 through the devious course of trial and error, at last 

 found rest in natural and fairly well-defined groups. 



OUR BOOK SHELF. 



Facts about Processes, Pigments and Vehicles j a Manual 

 for Art Students. By A. P. Laurie, M.A., B.Sc. 

 (London : Macmillan and Co., 1895.) 



In the majority of cases when a student of painting has 

 seriously entered upon his work in a school of art, he has 

 no wish, he makes no attempt to investigate the chemical 

 and physical properties of the materials he employs. He 

 is content to copy the practice of his teachers and fellow- 

 learners ; facility in working and immediate effectiveness 

 are all he demands. He may even go so far as to resent 

 the intrusion of science into the domain of art. To ask 

 a painter to study exhaustively the chemistry of the 

 materials and processes of painting would be unreason- 

 able, for a whole-hearted devotion to the prime business 

 of his life must be his first concern. Nor can an adequate 

 grasp of the difficult and varied, problems offered by- 

 pigments and vehicles and painting-grounds be acquired 

 by listening to a few lectures, witnessing a few experi- 

 ments, and reading a few chapters in a manual. The 

 author of the little book before us makes a very modest 

 demand upon the time and patience of the student of 

 painting. Here are no symbols and formulae to repel 

 the uninitiated, no tables of constants, no complex 

 theories of reaction and change. Mr. Laurie's readers 

 are first furnished with a set of easy experiments which 

 have been devised to show in an obvious way the nature 

 and treatment of the chief pigments and vehicles. Then, 

 in part ii., some notes on methods of painting in tempera, 

 fresco, water, and oil are given, while the volume con- 

 cludes with a glossary of pigments and a list of the 

 chemicals and apparatus needed for carrying out the 

 experimental work described in the earlier chapters of 

 the book. There' is one section of the volume which 

 seems somewhat incongruous — a description of "drawing 

 for process " and an endeavour to estimate the artistic value 

 of the leading methods of photographic reproduction. 

 Mr. Laurie will doubtless effect some improvements in a 

 second edition— a little more attention to literary style is 

 desirable. The late Mr. Gambier Parry of Highnam 

 Court would have been surprised to find himself described 

 as French. There are, indeed, very few slips or errors in 

 this little volume— very few statements and explanations 

 with which the writer of this notice does not agree. 



A. H. Church. 



