GROWTH OF GEOLOGY. 251 



When this is compared with a recent book on Physical 

 Geology, such as Prof. James Geikie's Earth Sculp- 

 ture, we are at once impressed by the fact that only a 

 few additional modes of operation have been discov- 

 ered in the course of the century. The progress has 

 been in measuring the efficacy of the factors which 

 Hutton recognised, rather than in discovering new 

 ones. 



A Case of Probable Uniformity. It is a fa- 

 miliar fact that water and air in various ways de- 

 nude the solid land, sometimes acting chemically, 

 as in the breaking up of silicates into insoluble and 

 soluble constituents, sometimes acting more me- 

 chanically in disintegrating without decomposing. 

 The insoluble results of denudation are deposited as 

 gravel, sand, and mud; the soluble constituents may 

 also be deposited (by evaporation, chemical action, 

 or through the agency of living creatures) to form 

 carbonates, sulphates, chlorides, or less frequently 

 oxides. This is a world-wide process, which prob- 

 ably went on in pre-Cambrian times very much 

 as it does to-day. " There is no evidence," says 

 Prof. J. J. H. Teall (now Director-General of the 

 Geological Survey of Britain), " that any of our sedi- 

 mentary rocks carry us back to a time when the physi- 

 cal conditions of the planet were materially different 

 from those which now exist." * 



Study of Volcanoes. The acrimonious contro- 

 versy between " Vulcanists " and " Neptunists," 

 which has been already referred to, dragged its 

 weary length into the first quarter of the nineteenth 

 century. The " Vulcanists," championed by Hut- 

 ton, upheld the igneous origin of such rocks as basalt ; 

 the " Neptunists," led by Werner, declared igneous 



* Address Section C, Rep. Brit. Ass. for 1893, p. 737. 



