Geology. 503 



ceeded by one devoted to a description of the various modes in 

 which igneous rocks occur as -geological bodies. 



The second part of the work considers the nomenclature and 

 classification. After a short historical sketch the author presents 

 a qualitative mineralogical classification which is based in essence 

 on the quantitative classification in that the rocks are divided 

 first into five groups, as follows: 1, chiefly quartz; 2, quartz and 

 feldspar; 3, feldspar; 4, feldspar and feldspathoid; 5, chiefly 

 feldspathoid. These are subdivided according to the nature of 

 the feldspars into A, alkalic feldspars; B, calci-alkalic feldspars; 

 C, soda-calcic feldspars, and each of these has a subdivision 

 according to whether there is much or little ferromagnesian 

 mineral ; a final group contains the rocks without feldspars. 

 Under these divisions the rocks, under the names ordinarily used, 

 are grouped, and while many names fall into certain compart- 

 ments it is interesting to observe that not many of the rock kinds 

 ordinarily recognized under the rather loose groupings now 

 employed, have to be split up. There is of course a further 

 division under texture and in the text explaining the table ceno- 

 typal and paleotypal habits are recognized. 



The volume closes with a full statement of the quantitative 

 classification as previously presented by the author and others. 



This volume in many respects, some of which have been indi- 

 cated above, is the most important treatise on the theoretical side 

 of petrography which has yet appeared. It should be in the 

 hands of every teacher and advanced student of the subject. 

 In comparison with the work of Mr. Alfred Harker, also 

 noticed in this Journal, it is interesting to observe the different 

 standpoints of the two men. While the subject matter covered 

 is essentially the same in each work, Professor Iddings's stand- 

 point is chiefly the physico-chemical one, while Mr. Harker lays 

 weight on the geological aspect. Thus to draw a compari- 

 son from biology, one work is chiefly anatomical, the other faunal, 

 in its viewpoint. Thus in a measure the two works supplement 

 each other. 



The typography, illustrations and general make-up of the 

 volume are excellent and a credit to the well-known firm of pub- 

 lishers which issues it. ~l. v. p. 



13. Natural History of Igneous Rocks ; by Alfred Harker, 

 8°, pp. 384; 2 pis., 112 diagrams. London, 1909 (The Macmillan 

 Co.). — The author states in his preface that the volume con- 

 sists in substance of the course of lectures delivered by him in 

 Cambridge University on petrology. He first considers igneous 

 action and igneous rocks from the purely geologic standpoint, and 

 emphasizes this aspect of the subject from the feeling that it has 

 not yet received the recognition it deserves as a part of historical 

 geology. In accordance with this we find the first chapter devoted 

 to igneous action, and after considering various regions the 

 author announces three distinct phases: (1) volcanic extrusions, 

 (2) plutonic intrusions, and (3) minor intrusions. When the 



