212 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



A large number of interesting observations have also been 

 made by geologists on the wear and 'tear that takes place on 

 broken rock-material in the course of its transport by a river. 

 Professor Daubree demonstrated experimentally the effects of 

 mutual abrasion. By subjecting fragments of granite and 

 other rocks to artificial means of trituration and friction, he 

 produced the rounded water-worn forms of pebbles and the fine 

 sand and mud characteristic of river detritus. He also 

 showed that the chemical action of the water appreciably 

 contributed to the dissolution of the fragments. The de- 

 position of the transported material over alluvial tracts at the 

 entry of rivers into fresh-water lakes and the ocean, was fully 

 and ably treated in the writings of De la Beche, Lyell, and 

 Elie de Beaumont. And since the publication of the earlier 

 works, the literature has been enriched by the special 

 contributions of Delesse, as well as by the excellent exposition 

 of the subject contained in the text-books of Geikie, De 

 Lapparent, Von Richthofen, and others. 



The speculative aspect of the invasions of the land by the 

 sea had been frequently dealt with in the writings of the 

 Greek and Roman philosophers. Careful historical records 

 had also been kept of the more striking changes in the 

 Mediterranean coast-lines. Von Hoff, in his account of the 

 inroads made by the sea, embodied all the previously known 

 data, both historical and scientific, regarding the mechanical 

 action of breakers, tides, and currents in the erosion of a 

 coast-line. New observations were added by De la Beche and 

 Charles Lyell ; and Oscar Peschel in his Physical Geography 

 (1879) discussed the particular form of coastal outlines in 

 their relation to the destructive action of breakers. 



While PeschePs views of the action were based upon a 

 supposed stationary condition of the coasts, Baron Richthofen 

 brought new life to bear on the subject when he pointed out 

 that the denudation of a coast may be going on contem- 

 poraneously with a movement of elevation or subsidence of 

 the land (China, vol. ii., 1882). In the former case, the 

 breakers of the retreating ocean can only erode a denudation 

 slope parallel with the original outline of the beach, and the 

 depredations of atmospheric weathering tend to rapidly 

 produce an irregular appearance of .the surface. As the 

 movement ceases, a marine terrace is formed, or if several 

 pauses occur at periodical intervals, a series of terraces is 



