56 NATURAL SCIENCE. Jan., 



commended, save the last, which, though clearly and well executed, is, 

 in its style (line-process), in painful contrast to the rest. We com- 

 mend them to the attention of the Malacological Society of London, 

 whose Proceedings are apparently not always illustrated by artists 

 who may be considered adepts in lithographic work. 



A Cambridge Manual of Petrology. 



Petrology for Students. An introduction to the study of rocks under the 

 microscope. By Alfred Harker, M.A., F.G.S. Svo. Pp. viii., 306. Cambridge: 

 University Press, 1S95. Price 6s. 



As the author remarks in his preface, there is room for a simple text- 

 book of petrology for the use of elementary students. Something is 

 wanted with more discussion of general problems than is to be found 

 in Cole's " Practical Aids," and more detailed and less a catalogue of 

 rock names than Hatch's " Petrology." The book that is really 

 wanted is a Rutley's " Study of Rocks " brought up to date. Mr. 

 Harker's book does not quite supply this, for he assumes more 

 preliminary knowledge in his reader. He opens with a short 

 introductory chapter referring to the use of the microscope, and the 

 principal optical characters of simple application, such as colour, 

 refractive index, extinction, and pleochroism. These subjects are 

 briefly treated, and the student is referred for a systematic treatment 

 of the subject elsewhere, and especially to Idding's translation of 

 Rosenbusch. The principal part of the work consists of a 

 description of the British rocks, grouped into the four divisions of 

 plutonic, intrusive, volcanic, and , sedimentary. The text bears 

 throughout marks of sound knowledge and careful preparation. The 

 descriptions of the rocks are clear, and as detailed as is necessary ; all 

 the principal variations from the main types are briefly referred to. 

 The literature has been summarised very carefully, and the series of 

 footnotes renders the book an index to British rocks. The section 

 on the sedimentary rocks (chapters xvi.-xix., pp. 192-253) is more 

 detailed than is usual in petrological works, and Mr. Harker is 

 especially to be commended for. his treatment of this interesting and 

 neglected subject. The last three chapters are devoted to the problems 

 of thermal and dynamic metamorphism, and to the description of 

 various crystalline rocks which do not fit conveniently into either of 

 the four classes. 



The book has several strong recommendations : the descriptions 

 are concise, the references to literature numerous and well-selected, 

 and the facts accurate. It will, therefore, no doubt prove of great 

 educational value. But in the absence of a glossary, the frequent use of 

 technical terms, such as " phenocryst," before they are explained, must 

 lessen its use to the solitary student. It has, however, apparently 

 been written for students who are attending a university course or its 

 equivalent, and to such it can be confidently recommended as a con- 

 cise and reliable manual of British rocks. 



A Cambridge Manual of Botany. 



The Elements of Botany. By Francis Darwin, M.A., M.B., F.R.S. Cambridge 

 Natural Science Manuals. Pp. xvi., 235. Cambridge: University Press, 1S95. 

 Price 6s. 



This book contains the substance of the botanical part of the course of 

 lectures in Elementary Biology given to Cambridge medical students. 

 But it is much more than this. It forms, without question, the best 

 introduction to the study of botany in the language. The lines on 



