ESSAY-REVIEWS 73 



known granites, which are often of gigantic size, are always 

 intrusive into such sediments. Clearly those granites are the 

 refused equivalents of pre-existing granite. The process of 

 stoping, fusion, and assimilation would advance the progress 

 of differentiation in the direction of more and more siliceous 

 granite, and such a progressive change, at least in the regions 

 personally known to the writer, has undoubtedly taken place. 

 On the other hand, if basalt magma has always been the source 

 of the later granites, these ought to be less siliceous than the 

 earlier ones, on account of the growing exhaustion of the 

 parent basalt magmas. 



The theory emphasised by Daly, and supported by Bowen, 

 that basalt magma is the parent magma of all igneous rocks is 

 probably true for most rocks. The source of heat and volatile 

 fluxes is, however, now to be sought at far greater depths than 

 those at which basalt magma can exist. Radioactivity, iso- 

 stasy, and tidal phenomena all unite in suggesting that fusion 

 must first take place at a depth of several hundred kilometres. 

 If, as is probable, the rocks at such a depth are peridotitic in 

 composition, then it would appear that only a long process of 

 differentiation, stoping, and assimilation could be adequate to 

 provide first basalt magmas, and afterwards granite magmas. 

 Peridotite itself is so heavy that only rarely would un- 

 differentiated intrusions of that composition succeed in reaching 

 the upper levels of the earth's crust. 

 Geol. Dept. Imperial College, London. 



STRUCTURE OP COAL, by Marie C. Stopes, D.Sc, Ph.D., 

 Fellow and Lecturer in Paleobotany, University College, London : on 



(1) The Origin of Coal, by David White and Reinhardt Thiessen, with 



a chapter on the Formation of Peat by C. A. Davis. Bull. 38 Bureau 

 of Mines, U.S.A. [8vo. Pp. 304, pis. liv. Washington. 1913.] 



(2) The Coals of South Wales, with Special Reference to the Origin and 



Distribution of Anthracite, by Aubrey Strahan, M.A., Sc.D., LL.D., 

 F.R.S., and W. Pollard, M.A., D.Sc, F.I.C., assisted by E. G. Radley. 

 2nd ed. Memoir Geol. Surv. England and Wales, 1915. [8vo. Pp. 91, 

 pis. x.] 



(3) Histoire Naturelle d'un Charbon (Russian and French), by M. D. 



Zalessky. Mem. Comite Gdol. nouvelle se"r. livr. 139. Petrograd. 

 [4to. Pp. 74, pis. xii.] 



These three papers, the work of leading men in the Geological 

 Surveys of the United States, this country, and Russia respec- 



