320 GEOLOGIC TIME. 



There is no evidence in tlie marine geologic formations of this conti- 

 nent that they were deposited in the deep sea; on the contrary, tliey 

 are nnlike such deposits and bear positive evidence of having been hiid 

 down in rehitively shallow waters. Limestones with ripple niiirks and 

 sun cracks occur, and beds of ripple-marked sandstones alternate with 

 shales and limestones. The more massive limestones, however, appear 

 to have accumulated in deeper water. The conditions in the Cordilleran 

 sea were, 1 think, more favorable for rapid deposition than in the deep 

 open ocean, but probably not as favorable as about coral reefs and 

 islands. The limestones and often the contained fossils clearly indicate 

 the presence of many of the same conditions of deposition as described 

 by the authors I have (pioted. More or less decomposed shells occur in 

 nearly every limestone; and a, large j)roportion of limestones, especially 

 the nonmetamorphic marbles, clearly shows that they were deposited 

 under the influence of the agencies at work in the laboratory of the sea. 

 Willis states that this occurs in the shallow waters of the Everglades of 

 Florida, and there is no a priori reason Avhy it did not occur throughout 

 geologic time; on the contrary, there is no doubt that it did. 



Rate of deposii informer times. — It has frequently been assumed that 

 in the earher epochs the conditions were more favorable for rajiid denu- 

 dation and inconsequence thereof the transportation and deposition of 

 sediment were greater. Prof. Prestwicli considers* that i^rior to the 

 sedimentary rocks the land surface consisted of crystalline or igneous 

 rocks subject to rapid decomposition owing to the composition of the 

 atmosphere and to their inherent tendency to decay. They must have 

 yielded to wear and removal with a facility unknown amongst mechani- 

 cally-formed and detrital strata where erosion operates. He thus 

 accounts for one of the factors that gave the large dimensions and 

 thicknesses of the earlier formations. Mr. Wallace thinks that geolog- 

 ical change was probably greater in very remote times,t stating that 

 all telluric action increases as we go back into the j)ast time and that 

 all the forces that have brought about geological phenomena were 

 greater.} 



Dr. Woodward says on the opposite view, that in the earliest geolog- 

 ical periods each bed of sand, clay, limestone, etc., had actually to be 

 formed, and that later deposits had the older sedimentary ones to furnish 



* Geology, 1886, toI. i, pp. 60, 61. 



t Jshoid Life, 2d ed., 1892, pp. 223-224. 



t Sir William Thomson (Lord Kelvin) inferred from his investigations npon the 

 cooling of the earth, that the general climate can not be sensibly alfected by con- 

 ducted heat at any time more than 10 000 years after the commencement of super- 

 ficial solidification. Treatise on Natural Philosophy, Cambridge, 1883, vol. 1, pt. 2, 

 p. 478. 



Of the degree of the sun's heat we know so little that conjectures in relation to it 

 have little force against the conditions indicated by the sedimentary rocks and their 

 contained organic remains. 



