March 27, 1914] 



SCIENCE 



467 



away beyond our ken in the structure of mat- 

 ter, say of copper, will confess that the chemist 

 can point to a certain set of properties which 

 are necessary and sufficient to delimit copper 

 from every other known substance. The ques- 

 tion here raised is similar, namely, Can forces 

 be grouped into a class by themselves ? And, if 

 so, what are the marks, or the one mark, by 

 which this class is set off from the other phys- 

 ical quantities ? Dr. Dadourian, in his " An- 

 alytical Mechanics," p. 15, has perhaps given 

 an answer to this question: but if so, only by 

 introducing a term — action — which the intelli- 

 gent reader will consider an undefined syn- 

 onym of force, equally complex and equally in- 

 definite.- Every one agrees that a force is 

 represented, in a general way, by a push or 

 pwll; but the question here raised is this : How 

 is a push to be defined in a quantitative and 

 consistent manner? 



Eeturning now from this digression sug- 

 gested by Professor Guthe's treatment of 

 mechanics, the definitions in Sound and Heat 

 are brief and excellent. Those in Magnetism 

 and Electricity are introduced with the inter- 

 esting remark that " the existence of ether in 

 space is accepted as a means of interpreting 

 phenomena that can not be explained by the 

 properties of ordinary matter." The defini- 

 tions which follow are especially fine and are 

 certain to furnish new and helpful viewpoints 

 to any serious student; the same is true of the 

 section devoted to optics. Where differences 

 of opinion might arise — and they are numer- 

 ous — one feels always that the text, as it 

 stands, clearly sets forth the essential facts 

 of the case. Henry Crew 



Introduction to the Study of Igneous Rochs. 



By George I. Finlay, Ph.D. New York and 



London. McGraw-Hill Book Company, Inc. 



1913. Pp. vii + 228. Price $2.00 net. 



This little book is said by its author to be 

 intended as an introduction to the exhaustive 

 treatises on the subject of igneous rocks, and 

 consists of a brief statement of the qualitative 

 classification of igneous rocks; a description 



2 See Rettger, Science, January 23, 1914, p. 140. 



of the method of determining such rocks in 

 hand specimens, and a short chapter on the 

 optical properties of minerals and the methods 

 by which they are determined. This is fol- 

 lowed by chapters on identification of the 

 essential and accessory minerals of igneous 

 rocks ; and by chapters on the " igneous type 

 rocks" and of varietal rocks related to the 

 type rocks; a brief synopsis of a method of 

 describing rocks; and an outline of the quan- 

 titative classification of igneous rocks, with 

 numerous examples of the method of calcula- 

 tion of the norm, with numerical tables to 

 facilitate the calculation. There are also 

 tabulated statements of the physical charac- 

 teristics of the chief rock minerals. 



The book is well gotten up and is to be com- 

 mended for its author's appreciation of the 

 value of quantitative methods of determination 

 and description, and for his simple and direct 

 manner of describing the ordinary method of 

 procedure in the customary identification of 

 rocks in hand specimens, and of minerals 

 under the microscope. 



It is a mistake, however, to call the book an 

 introduction to the more serious study of 

 igneous rocks as set forth in larger treatises 

 on the subject. It would seem to have been 

 prepared for a class of students who did not 

 intend to study the subject thoroughly, a very 

 large and legitimate class who desire only a 

 slight knowledge of the subject. Eor the work 

 labors under the disadvantage of an attempt to 

 simplify a highly complex subject, and to ex- 

 press in a few words ideas and definitions 

 which require fuller statements and amplifica- 

 tion in order to be correct. The attempt has 

 led the author into some errors that he might 

 have avoided. It has emphasized the idea of 

 rock types, which will lead students to expect 

 what they will not find in nature, and it has 

 given false ideas as to the composition of 

 rocks having the commonest names. The 

 author himself remarks that the concise state- 

 ment made in the table of igneous rocks on 

 page 98 may readily be misinterpreted by the 

 beginner. Why then make it? It certainly 

 conveys the impression that andesites are char- 

 acterized by mica and amphibole, and that 



