Reviews — F. P. MenneWs Manual of Petrology. 875 



The section on geology occupies twenty-eight pages, and when we 

 mention that it is written by Mr. F. W. Rudler it is needless to add 

 that it is an admirable essay. An account is given of all the 

 formations above mentioned, including even the ' Claygate Beds ', 

 a term recently applied to the passage-beds of sand and loam that 

 bridge over the interval between the mass of stiff London Clay and 

 the mass of overlying Bagshot Sands. Two photographic views are 

 given of the Sands in Ken Wood. The fossils of the London Clay 

 rightly receive a good deal of attention, and a new list is given of 

 species collected from the Hampstead Tube Eailway. The minerals 

 of the formations are duly noted, while the subjects of soils, scenery, 

 water-supply, and deep borings are not neglected ; nor, we may add, 

 are references to the former resident geologists, N. T. Wetherell and 

 Caleb Evans, and to others who have dealt with the geology of the 

 district. 



The section on topography is by Messrs. H. R. Maynard and 

 C. J. B. Findon, while one of the three botanical chapters, that on 

 the vegetation of Hampstead Heath and the neighbouring woods, by 

 Mr. A. G. Tansley, contains references to the soils and substrata, the 

 author observing that "it is essential to understand the geological 

 structure of a district in any attempt to unravel the distribution of 

 the vegetation ". In other chapters there are records and descriptions 

 of various forms of animal life, also of pond life in particular, and of 

 climate . 



IV. — A Manual of Petrology. By F. P. Mennell, F.G.S., etc. 



8vo; pp. 256, with 124 figures in the text. London: Chapman 



and Hall, Limited. Price 7s. &d. net. 

 rriHIS book forms an enlarged and improved edition of the author's 

 X Introduction to Petrology, which was published in 1909. The 

 text has been printed in larger type, and over fifty of the figures, 

 mostly photomicrographs, are new and are a great improvement on 

 those which appeared in the book in its earlier form. The chapters 

 have been re-arranged in more logical sequence, several of them have 

 been rewritten, and tables of analyses have been insei'ted. 



The first three chapters deal with the general and optical properties 

 of rock-forming minerals. Due emphasis is laid on the value of 

 interference figures as aids to the determination of minerals in thin 

 sections. For purposes of description the minerals are classified 

 according to their chemical compositions, instead of by their optical 

 properties. Had they been arranged according to the latter this part 

 of the book woTxld have been more immediately useful in determinative 

 work. The insertion of the actual values of the birefringence for the 

 doubly refracting minerals would have been a great improvement. 

 The simplification of the chemical formulae for one or two of 

 the minerals has been carried too far. Thus topaz is given as 

 Alg Og . Si Og, thereby failing to show the importance of the hydroxyl 

 and fluorine in its constitution. 



The descriptions of the igneous rocks occupy four chapters and 

 though necessarily short they are very much to the point. The 

 nomenclature has been reduced to the simplest form. The rocks are 



