Revietvs — Dr. F. H. Hatclis Petrology. 177 



forms the main construction of the Arenigs, the Aran, the Gader group, 

 and the Berwyns ; and in it we have the Festiniog, Corris, Aber- 

 gynolwyn, and Aberllefenni slate quarries. 



The Silurian Series stretches from near Bala to the eastern borders 

 of the county. The term ' Bala Beds ' is given to these rocks all over 

 AVales, because they are best developed from Dinas Mawddwy by 

 Bala to Bettws-y-Coed. (p. 36.) 



The largest 'fault' in the British Isles cuts through the middle of 

 Bala Lake from south-west to north-east. Professor Sir Andrew 

 Kamsay has made the Bala Lake historic by his description of its 

 glacial origin ; indeed, the whole region is classic ground to the geologist. 



In addition to its scenery (unsurpassed by any other county in 

 Wales) Merionethsliire is full of interesting relics and antiquities. 

 It is rich in churches, abbeys, castles, manor houses, Roman remains, 

 whilst its survivals of Welsh manners, habits, and customs exceed 

 all the other counties of Wales and would alone suffice to attract the 

 intelligent visitor. 



Mr. Morris's handy County Geographj^, from its small size and 

 completeness, will be found well suited for a pocket companion. 



II. — Text-book of Petkologt, Voltime I. 

 The Petrology of the Igneous Bocks. By F. H. Hatch, Ph.D. 

 7th edition, revised. Cr. 8vo; pp. xxiv-|-454, with 154 figures. 

 London : George Allen & Co., Ltd., 1914. Price 7s. Qd. net. 



LAST year Dr. Hatch and Mr. R. H. llastall published the Petrology 

 of the Sedimentary Rocks, styling it " Text-book of Petrology, 

 Volume II ". It was then understood that a volume uniform with 

 this and dealing with igneous rocks would appear shortly : this volume 

 is now before us in the form of a revised edition of Dr. Hatch's 

 "Text-book of Petrology", first published in 1891, but almost 

 entirely rewritten in 1909. The two volumes form a textbook which 

 claims to constitute a brief but complete summary of the whole 

 science of rocks. 



In revising the book the author has had occasion to write two new 

 chapters on Pyroclastic Bocks and on the metaraorphic derivatives of 

 igneous rocks, the latter really formiug a supplement to the more 

 general treatment of metamorphism found in the Petrology of the 

 Sedimentary Racks. Here the metamorpliosed igneous rocks are 

 classed as: plutonic gneisses, schists derived from hypabyssal, from 

 volcanic, and from pyroclastic rocks ; such a classification can only be 

 applied when the origin of the metamorphic rocks can be ascertained. 

 For the subdivisions of the plutonic gneisses the five groups suggested 

 by Dr. Teall in 1907 are used. These groups are based primarily on 

 mineralogical constituents and to a subordinate extent on structure, 

 and, in the present state of our knowledge, they form most useful 

 divisions. By using the mode of origin as a basis of classification in 

 this chapter Dr. Hatch has compelled liimself to omit some interesting 

 types of metamorphic rocks M'hich were not dealt with under the 

 sedimentary rocks. Thus we look in vain for any account of the 

 granulites, though the more basic pyroxeue-granulites appear under 

 plutonic gneisses. 



DECADE VI. — VOL. I. — NO. IV. 1"- 



