252 
[March, 
Notices of Books . 
Bi 2 0 3 .N 4 0 I0 .” It is afterwards applied to various minerals de- 
rivable from two silicic hydrates, of which the second is the 
cumulate of the first. 
The present volume goes down to the end of F : it embraces 
all new matters of any importance in Inorganic and Organic 
Chemistry, and in Mineralogy. We do not notice any articles 
on physical or technical subjects, and the great mass of matter 
relates to Organic Chemistry. 
An Elementary Text-Book of Petrology. By Frank Rutley, 
F.G.S. London : Longmans and Co. 
The study and recognition of rocks may fairly be considered as 
the alphabet of geology, without a knowledge of which the 
student may certainly become a reader, but never a worker in 
the science. In other words, he may peruse handbooks and 
manuals, may fix their contents in his mind, and pass an exami- 
nation with credit, and yet be utterly unable to make an original 
trustworthy observation or to verify the theories which may be 
put forward. It is therefore strange that certain geological text- 
books— to particularise would be invidious — overlook this part of 
the subject altogether. The reader is supposed to be born with 
the power of recognising the respective species of rocks, and 
therefore at the end finds himself no more of a geologist than he 
was at the beginning. Mr. Rutley, in the text-book before us, 
makes a useful and praiseworthy attempt to supply the know- 
ledge thus found wanting. Into chemical methods for the 
diagnosis of rocks and their constituent minerals he does not 
enter at length, the rather as all needful information on this head 
may be found in well-known and accessible works. He occupies 
himself the rather with physical, and especially optical, charac- 
teristics. The microscope — thanks to the exertions of the late 
David Forbes, of H. Witham, and especially of Mr. H. C. Sorby 
— is now no less essential to the geologist than to the stu- 
dent of animal and vegetable life, and our author explains 
clearly and fully the applications of this instrument in petro- 
logical research. 
We can strongly recommend this work to all who are entering 
upon the study of geology, and who wish at the outset to lay a 
firm foundation. 
