1 84 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



" The materials composing pelagic deposits are not directly 

 derived from the disintegration of the continents and other 

 land-surfaces. They are largely made up of the shells and 

 skeletons of marine organisms secreted in the surface-waters of 

 the ocean, consisting either of carbonate of lime, such as 

 pelagic Molluscs, pelagic Foraminifera, and pelagic Algae, or of 

 silica, such as Diatoms and Radiolarians. The inorganic con- 

 stituents of the pelagic deposits are for the most part derived 

 from the attrition of floating pumice, from the disintegration 

 of water-logged pumice, from showers of volcanic ashes, and 

 from the debris ejected from submarine volcanoes, together 

 with the products of their decomposition." (Sir John Murray, 

 Brit. Assoc., 1899.) 



Throughout the earlier parts of the nineteenth century much 

 labour was expended on the description of different parts of 

 the continents, but the treatment was too formal to advance 

 the conceptions of the connection between the physiography 

 and geology of the earth. A desire gradually made itself felt, 

 not only to describe, to measure, and to compare the actual 

 forms and to follow their distribution, but also to explain their 

 origin and development ; and the two sister studies more fully 

 recognised their community of aim. The physical exposition 

 of the Swiss Jura mountains by Thurmann, in 1832, gave a 

 strong impulse to the new direction of thought in Europe, but 

 it was in the wide plateaux of America that the first signal 

 successes of physiographical geology were won. The brilliant 

 works of Dana and Leslie were followed by those of Powell, 

 Dutton, Gilbert, and other pioneer geologists in the Far West; 

 by their vivid portrayal of the work of subaerial denudation 

 the American writings roused the intellectual life of the middle 

 of the century to new conceptions on a grand scale. 



' The gigantic erosion forms in the Bad Lands, the configura- 

 tion of the Rocky Mountains and of the plateaux lands in 

 Arizona, Colorado, and Mexico, the wonders of the Yellow- 

 stone Park and California, called forth a new and rich literature, 

 which demonstrated in. the most convincing way that the sur- 

 face-forms of those regions are mainly the -result of the erosive 

 activity of water. 



Davis, MacGee, Chamberlin, and others have worked along 

 the same lines in the east of North America and the middle 

 States, where ice rather than water takes the first rank as the 

 agent which sculptured the prominent surface-forms. 



