1875.j Notices of Books. 109 
Les Rochers ; Description de leurs Elements ; Methode de Deter- 
mination. Par EpouaRD JANNETAZ. Paris: J. Roths- 
child. 
ELEMENTARY works on geology are plentiful in the languages of 
all civilised nations, or cultur-vélker, as the German phrase 
goes. These manuals give a description of the various rocks ; 
treat of their occurrence and localities, their chemical compo- 
nents and organic remains; their supposed origin and age, and 
the changes they are inferred to have undergone from volcanic 
or from glacial action. But one thing is wanting. The tyro, 
who does not wish to limit his studies to books, but to become 
himself a reader of the great stone documents, asks ‘‘ How am I 
to distinguish the various species of which you speak?” To 
this question ordinary geological text-books return no definite 
answer. The student may go to the localities indicated for any 
particular formation, and may there study its characteristic 
features, so as to recognise it on any future occasion. M. 
Jannetaz, however, endeavours to supply a direct answer to the 
question, and to give instructions for the determination of rocks. 
In the first part of the work we find a description of the 
‘“‘mineral species most important from a lithological point of 
view,” with their chemical, optical, and physical characters, the 
crystalline form being in most cases shown ina diagram. The 
second part treats of the rocks themselves, their essential and 
their secondary or accessory constituents. The section on 
granite gives a characteristic instance of the manner in which 
this department is worked out. 
Lastly, we come to the ‘‘ method to be followed in the practi- 
cal determination of rocks,” a portion of which we transcribe, to 
give the reader an idea of its nature :— 
Sect. 1.—Globular rocks (p. 226). 
I. Globules (A. Vitreous globules. 
harder than}B. Crystalline ditto. 
the point of a)C. Irregular grains, without intervening matter. 
graver, p. 226.\D. Globules forming mamillary masses. 
1. Effervescing in HCl, and not becoming mag- 
netic on charcoal. 
“Lo eines 2. Effervescing in HCl, and becoming magnetic. 
3. Not effervescing, but becoming magnetic. 
4. Not effervescing, and not becoming magnetic. 
softer, p. 228. 
Each of these heads is then further extended, until the clue 
followed leads the enquirer to the species sought. 
We are perfectly aware that in no department of natural 
science is there a royal road to identification of species. When 
systematists have done their best, much will still be left to the 
patience and the tact of the student. We know that the very 
