382 STUDIES FOR STUDENTS 



First, marine conglomerates and sandstones. — Due to marine plana- 

 tion and transportation, enabled to reach wide horizontal extent over 

 shallow seas through crustal movements shifting the zone of wave and 

 current action. 



Second, tectonic conglomerates and sandstones. — Due to subaerial 

 erosion owing to a steepening of the river slopes, either from mountain 

 making, crUstal warping, or subsidence of the ocean level. 



Third, climatic conglomerates and sandstones. — Due to climatic 

 change without necessarily any new accompanying crustal deformation. 

 In distant periods, when the change has been of such a nature that the 

 accumulations of gravel and sand were local and deposited near the 

 sources of the material there is less probability, on account of the 

 smaller original area and its higher level of deposition, of such being 

 observed, or separated and distinguished from deposits of purely 

 tectonic origin made by erosion during a period of stable climate. 

 When the change was of such a nature, however, either to a more 

 rainy or colder climate, that the pebbly or arenaceous detritus was 

 swept forward for hundreds of miles from previous sources of erosion 

 and accumulation, formations which may be described as great sand- 

 stone plates must have resulted, intercalated between others of a finer 

 and more argillaceous nature, conspicuous in the geologic column both 

 from their areal extent and their physical contrast; a contrast due to 

 the climatic change operating in the region of deposition as well as 

 influencing the character of the transportation. Applying this prin- 

 ciple to the geologic past, if the sections made by nature across the 

 deposits of ancient deltas or shallow seas indicate a periodical fluctua- 

 tion in kind and in coarseness of sediment, and if such variations can 

 be correlated on other grounds with climatic changes of the proper 

 nature, it is the simpler hypothesis to hold that the latter and not 

 repeated synchronous deformations have been the causes of the 

 changes in sedimentation. 



In the history of geologic science it is to be noted that to marine 

 action was once ascribed the greater part of planation of the land and 

 the formation of conglomerates and sandstones from the debris; a 

 natural stage in geologic theory, when it is considered that the earth 

 science developed most largely upon the northwestern shores of Europe 

 where there is a maximum of coast line, of coastal erosion, and of 

 shallow sea. An awakened appreciation, however, by the following 



