Revieics—Dr. F. H. Hatch's Text- Book of Petrology. 177 



devised for the discrimination of the mineral species constituting 

 a rock-section, and the resulting discoveries have led to not in- 

 significant changes in the classification of rock species and to greater 

 refinement in subdivision. As the years passed it became increasingly 

 evident that a complete revision was necessary if the work was to 

 retain its former position among petrological textbooks. This 

 has now been done, and the present edition may confidently be 

 recommended to the student who requires a sound, yet not too 

 abstruse, grounding in the characteristics of the several species of 

 igneous rocks, and in some of the simpler methods for their dis- 

 crimination. Examples have been freely drawn from recent research, 

 such as, for instance, Harker's admirable memoir upon the Tertiary 

 igneous rocks of Skye. 



While the general framework of the book remains much the same, 

 there is scarcely a paragraph that has not undergone alteration. 

 In the opening pages the student who is desirous of wider reading- 

 will find a list of the principal works on the subject. In Part I 

 the author discusses the physical characters of rocks, such as their 

 mode of occurrence, their macro- and micro-structure, and their 

 composition. In Part II he deals with the rock-forming minerals 

 and their characters. Some space is devoted to the phenomenon 

 of double refraction, upon which depends the usual method of 

 distinguishing the minerals seen in a thin section, and a list is given 

 of the principal rock-forming minerals, arranged according to their 

 maximum birefringence. Since petrological students generally acquire 

 some knowledge of mineralogy and crystallography, it would have 

 been advantageous if the refractive properties had received somewhat 

 fuller treatment. In the description of the minerals the mean 

 refractivity is given, and the double refraction is characterized as 

 'strong', 'moderate', or 'weak': it would have taken no more 

 space to have stated the greatest and least of the refractive indices, 

 and the information would have been fuller and more complete. In 

 the description of the mineral species attention is, of course, chiefly 

 paid to their appearance as seen in rock-sections. In Part III we 

 find a clear and lucid discussion of the igneous rocks, arranged 

 according to the plutonic, hypabyssal, and volcanic groups. The 

 characters and distinguishing features of each family are pointed 

 out, typical analyses are quoted, and happy use is made of Iddings' 

 elegant diagrammatic method for representing the composition of 

 the rocks constituting a family. The reproductions of micro-photo- 

 graphs, prepared by Mr. R. H. Rastall from sections in the Students' 

 Series in the Sedgwick Museum at Cambridge, facilitate the under- 

 . standing of the text. The account of the distribution of igneous 

 rocks in the British Isles, which was previously included in this 

 part, has now been expanded and forms Part IV. This feature of 

 the book renders it peculiarly valuable to students in this country. 

 A useful table, prepared by Mr. P. H. Pastall and Mr. J. Romanes, 

 for the determination of the minerals in a "thin section by means 

 of their optical properties is appended, and a full index brings the 

 work to a close. 



Dr. Hatch has endeavoured to make the subject intelligible to 



DECADE v. VOL. VI. — NO. IV. 12 



