DYNAMICAL GEOLOGY. 21$ 



found many evidences leading to the conclusion that there 

 had been enormous denudation of Jurassic deposits in certain 

 areas. These few examples suffice to show how cautiously 

 one must use the present disposition of geological forma- 

 tions as a basis for the reconstruction of maps portraying 

 the distribution of continent and ocean in past geological 

 epochs. It is almost impossible in the case of the older sedi- 

 mentary deposits to ascertain the amount of denudation they 

 have incurred in past ages. 



Mechanical Sediments in ihe Ocean. In the eighteenth 

 century, De Maillet had investigated the deposition of sediment 

 on the floor of the ocean. Early in, the following century, the 

 writings of De la Beche, Lyell, and Elie de Beaumont provided 

 able chapters on sedimentation, and explained the deposition 

 of detritus over alluvial tracts, and on the floor of fresh-water 

 lakes, inland seas, or the ocean. The observations of these 

 authors were made chiefly on the English, French, and Medi- 

 terranean coasts. 



A classical work on the subject, The Lithology of the 

 Sea-Floor, was published in 1871 by the engineer and geologist, 

 M. Delesse. Beginning with a full exposition of the origin 

 and constitution of the material transported from Continent to 

 Ocean, Delesse next describes the sediments throughout the 

 whole sea margin of France, and then depicts those in the 

 other seas of Europe and along the coasts of North and Central 

 America. Three coloured maps show the distribution and the 

 petrographical character of the marine sediments in these 

 areas, and illustrate for the first time the great variety in the 

 nature of the deposit on one and the same coast. Delesse, 

 applying his knowledge of the modern formations of sediments, 

 was enabled to reproduce in cartographical form the probable 

 distribution of land and sea in France during the Silurian, 

 Triassic, Liassic, Eocene, and Pliocene periods. Rough 

 sketches of a similar kind had been previously prepared by 

 Elie de Beaumont, by Lyell and Dana. Those of Delesse 

 have been a model for all subsequent efforts in this direction, 

 and have never been surpassed. The Atlas by Canu, pub- 

 lished in 1895, provides more geological detail, but the maps 

 are less clear. 



While the work of Delesse comprises all the important facts 

 known up to the year 1871 about the constitution of littoral 



