514 Reviews — A. Harker's Petrology. 



be distributed into kinds by means of litbical cbaracters alone," and 

 states that these may be grouped in regard to Mineral composition 

 and Structure. He takes thirty different rocks, and seeks to ascertain 

 how they may be classified. Seven preliminary groups ai*e first 

 made, but they lead to assemblages which are more curious than 

 instructive. Thus, among " Rocks essentially composed of material 

 belonging to a single mineral species," we find Rock-salt, Quartz- 

 rock, Limestone, and Serpentine grouped together; and again, among 

 " Rocks which, though really composed of material not belonging to 

 a single definite mineral species, are so far homogeneous in aspect 

 that the essentiality of the compositeness is concealed from the 

 unaided eye," we find Coal, Clay, Obsidian, Phcnolite, and Felstone 

 arranged together. We may, indeed, question the value of such 

 temporary assortments. It is as if we arranged the books in a 

 library according to their bindings ; and the author himself proceeds 

 to show that any arrangement based on apparent simplicity is 

 unsatisfactory for purposes of classification. He then discusses the 

 modes of origin of the rocks belonging to his seven preliminary 

 groups, and also the processes by which the rocks have attained 

 their present characters. This leads to certain re-arrangements in 

 his groupings, and to a number of separate descriptions of tlie 

 principal rocks. Hence the student must turn from page to page 

 if he wants to learn all that is said about crystalline schist, gneiss, 

 slate, shale, or any other rock. The index will here prove very 

 serviceable, but we venture to hope that in the future editions of his 

 work the author may find ways of simplifying his groupings and 

 descriptions of rocks. He has devoted great care and pains to his 

 work, he has given us much valuable and precise information, and 

 has pointed out a number of ways in which rocks may be arranged ; 

 but we believe the student would have found the path to know- 

 ledge easier if one general classification had been given, and if the 

 descriptions of each type of rock had been in sequence. 



Like all the British Museum publications, this work is excellently 

 printed, and in respect both to type and paper it sets an example 

 that might well be followed by Her Majesty's Stationery Office. 

 Its price is sixpence. 



IIL — Petrology for Students : An Introduction to the Study 



OP Rocks under the Microscope. By Alfred Harker, 



M.A., F.G.S. (Cambridge, at the University Press). 



rriHIS is a book which presents no easy task to the reviewer ; 



I it is so full of thorough and useful work and so carefully 



put together that there is practically nothing in it to correct and 



little to add, while to endeavour to make an abstract of it would be 



to make an extract of Liebig or to compress pemmican. Avoiding 



the long description of the petrological microscope which usually 



occupies so many of the first pages in a book of this kind, referring 



to other special works for descriptions of minerals, and laying 



aside any digression on methods of research other than study in 



the field, with the lens, and with the microscope, the author plunges 



