386 



NATURE 



\_August 23, 1888 



the chief phenomena revealed, when either plane or con- 

 vergent polarized light are employed, are absolutely indis- 

 pensable. But in addition to these there is a vast mass of 

 knowledge, which has been gradually acquired and is 

 ever increasing, concerning the internal peculiarities of 

 minerals, especially such as appear in the varieties that 

 constitute rocks, and with respect to the wonderful series 

 of changes which they undergo when exposed to different 

 conditions ; and the more of this kind of knowledge the 

 student can bring to the investigation of a rock the less 

 liable will he be to fall into error. In this branch of 

 science, as in every other, the experience which can only 

 be obtained by long-continued study of the subject must 

 always supplement, and may sometimes even supersede, 

 the results obtained by the application of rigid rules of 

 of procedure. 



As a suggestion has recently been made in the 

 pages of Nature that all which is required to secure 

 a uniform and uniformly-acceptable classification and 

 nomenclature of rocks is that some master of the 

 modern methods of research should bring in a sweeping 

 " reform bill " on the subject, it may be well to quote the 

 author's views upon petrographic notation and classifica- 

 tion. Writing after the two years of careful labour 

 devoted to the preparation of this work, he remarks : — 



" As regards the classification of rocks, I am sorry to 

 say that increasing knowledge has not tended to bring 

 about any clearness of view. The more rocks are studied 

 the less they seem to me to adapt themselves to any 

 classification at all comparable in definiteness with the 

 classifications of organic bodies and mineral substances. 

 Rock-masses often vary so much in composition and 

 structure that any scheme of classification based on work 

 done in the laboratory is unsuitable for the expression of 

 broad geological facts. It is absolutely impossible to 

 map the different varieties recognized by modern petro- 

 graphers. The conclusion at which I have arrived is 

 that the necessity for giving names to rocks arises rather 

 from work done in the field than from work done in the 

 laboratory. Rock specimens are mineral-aggregates, and 

 may be described as such. Rock-masses are integral 

 portions of the earth's crust, and possess a certain 

 amount of individuality in virtue of their mode of 

 occurrence." 



With these remarks we very cordially agree. Sys- 

 tematic mineralogy is a branch of natural-history science ; 

 for, in their crystalline forms and chemical constitution, 

 minerals supply safe criteria which enable us to define 

 species and varieties, and also permit us to group these into 

 larger divisions. But most petrographical classifications 

 seem to be of value only so long as we confine our atten- 

 tion to the selected fragments that fill the cases in a 

 petrographical museum. In the field one type is often 

 found passing into another which the mere petrographer 

 may have placed in a totally different class. 



There is perhaps just now a danger of our exaggerating 

 the importance of the microscopic method as applied to 

 the study of rocks. That the method has already done 

 much in enabling us to follow out and trace the effects of 

 the slow processes of change within the earth's crust, and 

 that it will do still more in the future, no one can doubt. 

 But when' it is sought to make the microscope a "court 

 of final appeal" in geological questions, and in doing so 

 to disregard the importance of field-observation, we per- 

 ceive the same source of danger as is now perhaps being 



experienced in connection with almost every branch of 

 natural-history research. It must be remembered that, 

 while the microscope enables us to see a little more than 

 the naked eye or the pocket lens, yet nevertheless, 

 between what is actually seen by the very highest powers 

 of our microscopes and the molecular groupings and 

 reactions which give rise to the varied phenomena of the 

 mineral kingdom, there is room for almost infinite possi- 

 bilities. We accept the teaching of the microscope with 

 all thankfulness, but we recognize the fact at the same 

 time that it has enabled us to get only a very little 

 nearer to the heart of those great physical problems 

 which we aim at solving. 



In congratulating the author upon the completion and 

 publication of a book which, as we learn from his preface, 

 has occasioned him no little anxiety as well as so much 

 labour, we may express the hope that his project of treat- 

 ing the aqueous and metamorphic rocks in the same 

 attractive and thorough fashion may be realized. We 

 cannot conclude this notice without a word of com- 

 mendation for the excellent glossary of terms used in 

 describing rocks, which has been supplied by Dr. F. H. 

 Hatch, and will, we are assured, prove of the greatest 

 service to students. John W. Judd. 



SILKWORMS. 

 Silkworms. (" Young Collector Series.") By E. A. 

 Butler, B.A., B.Sc, Author of " Pond Life: Insects," 

 &c. (London : Swan Sonnenschein, Lowrey, and Co , 

 1888.) 



THE silkworm is so familiar an insect to everyone, and 

 is interesting from so many points of view, that we 

 gladly welcome this small volume from the pen of a well- 

 known writer on popular natural history. The space 

 which can be allotted to this subject in works on general 

 zoology, or even on general entomology, is necessarily 

 small ; and when we consider that a whole library 

 could be written on the history and structure of any 

 single insect, a book dealing almost exclusively with 

 Bombyx mori should be a useful addition to our ento- 

 mological literature. The present work is fairly com- 

 prehensive in its scope, and is written in such a manner 

 as to be intelligible to everyone, however ignorant of 

 natural history. Numerous woodcuts are added, where - 

 ever they seem to be required to elucidate the text. 



Mr. Butler appears to be adequately acquainted with 

 his subject, and we have glanced through his book with- 

 out noticing any very serious errors, or meeting with many 

 statements which we felt disposed to question. But we 

 can hardly accept the inconceivable narrative which 

 Mr. Butler has copied from the Entomologist on 

 pp. 78 and 79, about a male and female moth being 

 developed upside down in a single pupa formed by a 

 single larva. Until more instances of a similar nature 

 are recorded, we fancy that most charitably-disposed 

 people will be inclined to imagine that some extraordinary 

 error must have occurred. In this case, and in a few 

 others, Mr. Butler quotes his authorities. Although it 

 would be unfa'ir to expect the author of a work like the 

 present to quote authorities throughout, we think that it 

 would have been more satisfactory to Mr. Butler's readers, 

 especially to those who may wish to go further into the 

 subject, if he had indicated in a brief preface the chief 



