Revieivs — Prof. Cole's Practical Geology. 231 



cbaracters and varieties of several groups — the coarsely fragmental, 

 ordinary stratified, cleaved, foliated, and igneous — being successively 

 i:)ointed out. The determination of the specific gravity, fusibility, 

 and chemical composition of rocks, is next considered ; and an 

 important chapter on the isolation of the constituents of rocks is 

 followed by a short and practical discussion of the petrological 

 microscope and the preparation of rock-sections. With the chapter 

 on the more prominent characters to be observed in minerals in 

 rock-sections, and the following synoptical remarks on the appearance 

 of thin sections of the chief rock-forming minerals, the student 

 enters upon a subject that requires long practice for its adequate 

 comprehension; but Prof Cole's "aids" are among the most valuable 

 that have hitherto appeared, including, indeed, some items that have 

 scarcely yet reached ordinary handbooks, and numerous notes in 

 special type when there is any possibility of doubt or misinterpreta- 

 tion. The sedimentary rocks, as usual, occupy a comparatively 

 small amount of space, but all the latest discoveries relating to them 

 seem to be duly incorporated in the synopsis. The igneous rooks 

 are treated with remarkable elaboration, and students will be grateful 

 to the author for presenting so concise a summary of the main facts 

 with a comparatively simple nomenclature. As Prof. Cole remarks, 

 " petrography has of late suffered from the introduction of an 

 abundance of new terms, and, what is far worse, of old terms defined 

 in new senses ; but the majority of these can be avoided by the use 

 of familiar arljectives or mineral prefixes, to the great lightening of 

 the science." The Holocrystalline Igneous Rocks are subdivided 

 into the six groups of Granite (with Eurite), Syenite, Quartz- 

 Diorite (with Quartz- Aphanite), Diorite (with Aphanite), Olivine- 

 Gabbro (with Olivine-Dolerite), and Peridotite. The Lithoidal 

 Igneous Eocks, with some glassy matter, are classified as Rhyolites, 

 Trachytes, Rhyolitic Andesites, Andesites. Olivine-Basalts, Lim- 

 burgites, and Hemicrystalline Nephelinites (with Leucitites). The 

 Highly Glassy Eocks comprise the Obsidian and Tachylyte Groups. 

 In the case of every terra, a reference to its original definition is 

 given, with frequent notes upon subsequent changes in its meaning ; 

 and a table is added exhibiting the relationships of the various types, 

 so far as determined. 



The chapter on Metamorphic Eocks is characterized by much 

 caution in alluding to theoretical matters, and the definition of the 

 limits within which the term " metamorphic " is applicable is well 

 stated to be a "matter of opinion." Prof. Cole places in this 

 category all rocks in which new crystalline developments, or new 

 structures, or both, have arisen imder the influence of subterranean 

 heat, or pressure, or actual earth-movement. He seems, moreover, 

 to show more sympathy with the original views of " regional 

 metamorphism," than with the recent speculations of those who 

 regard the x^rcheean rocks as uniqi;e among geological formations. 

 In referring to the so-called " true schists," for example, he remarks 

 " that the alleged distinction between schist-like rocks and schists 

 of Pre-Cambrian age requires such great delicacy of definition that 



