Reviews—JI. J. Harris Teall’s Petrography. 367 
significance attached to them by various workers. Succeeding 
chapters summarize the chemical and physical characters of igneous 
rocks, giving a brief notice of the uses made of certain chemical 
solutions in separating rock constituents, and determining their 
specific gravity. Indeed, we may say that in every part Mr. Teall 
appears to have brought up his information to the latest date, 
though he wisely abstains from burdening his book with long 
descriptions of methods of investigation, chemical or physical. 
Next comes the classification of igneous rocks, where the principle 
already noticed is enunciated. The author also discusses the vexed 
question of geological age as a primary factor in classification of 
igneous rocks. Here British petrologists have for some years been 
at issue with most of their fellow-workers on the Continent. Of the 
ultimate result of the contest there can now be little doubt. Mr. 
Teall takes this position, “‘ While declining to accept geological age 
as a primary factor in classification, . . . the present writer is 
strongly of opinion that, if possible, it should receive indirect ex- 
pression. . .. So when any peculiarity of texture or composition 
can be shown to characterize rocks of a particular period, that 
peculiarity should be utilized for purposes of classification.” To 
this concession no reasonable objection can, in our opinion, be made: 
probably, however, it will be rarely of avail in practice. The 
succeeding chapters describe the more important varieties of igneous 
rocks. This portion of the book will be found by the student a 
perfect mine of valuable information very lucidly arranged. Mr. 
Teall has mastered the voluminous literature of the subject, and 
gives an admirable summary of the result of his studies. His un- 
wearied patience and assiduity will cause future workers to invoke 
blessings on his name, and to save them from being crushed, to use 
Professor Huxley’s metaphor, beneath the gifts which have, been 
heaped upon them; gifts, among which, as in the case of Tarpeia, 
there is much metal of greater weight than worth, as well as the 
gold. This sketch is to a certain extent critical as well as historical, 
as it should be, but it is executed, as a rule, in a thoroughly judicial 
spirit. If the author shows any bias, it isin an occasional disposition 
to regard dynamic metamorphism too much as an established 
theory rather than a probable hypothesis. Perhaps also it would 
have been well to have mentioned that in some cases where foliation 
is claimed as the result of regional or dynamic metamorphism, 
difficulties have been indicated and other hypotheses suggested. 
Still, this potent factor of change has been so overlooked of late 
years, that we are glad to see the student’s attention called to it, 
It must hold a prominent place in future among the agencies of 
metamorphism, though we doubt whether Ecpiesis will ultimately 
enjoy a supremacy “quite as universal as seems at present to be 
claimed by some of her newly-converted devotees! 
Under each principal group the author describes the different 
examples and varieties of igneous rocks, according to their 
geographical distribution, thus adding to the utility of his book as 
oO 
a work of reference. There is a particularly valuable sketch of the 
