CLIMATES OF GEOLOCIC TIME. 273 



display. The evidence of the tiUites is in favor of the view that glaciation in Huronian 

 Canada was not "the work of merely local mountain glaciers," but rather due to "the pres- 

 ence of ice sheets comparable to those which formed the Dwj^ka. * * * This implies 

 that the climates of the earlier parts of the world's history were no warmer than those of 

 later times, and that in Lower Huronian times the earth's interior heat was not sufficient to 

 prevent the formation of a great ice-sheet in latitude 4G°" (192). (For further detail 

 see pp. 295-6.) 



CLIMATIC EVIDENCE OF THE SEDIMENTS. 



During the past ten years it has become evident that the color of the delta deposits of 

 geologic time, and esijecially that of continental deposits, is to be connected largely with 

 differences in climate. This evidence, however, is as yet difficult of interpretation, because 

 the climatic factors are not easily separated from those due to topographic form. All that 

 can be done now is to call attention to the marked changes in sedimentation from the 

 gray, green, blue, and black colors to the red beds which are so often also associated with 

 coarser materials. Barrell states : 



" The changes from the red beds of the Catskill formation, several thousand feet in thickness, 

 to the gray Pocono sandstones with a maximum thickness of 1,200 to 1,300 feet, then to the 

 sharply contrasted red shales and sandstones of the Mauch Chunk, 3,000 feet in maximum 

 thickness, and back to the massive white conglomerates of the Pottsville conglomerate, 1,200 

 feet in maximum thickness, followed by the coal measures, are all the result of increasingly wide 

 swings of the climatic pendulum which carried the world from Upper Devonian warmth and 

 semi-aridity to Upper Carboniferous coolness, humidity, and glaciation" (1908: 103). 



In regard to the significance of gray to black formations Barrell states: 



"Where a whole formation, representing an ancient floodplain or delta, shows in its un- 

 weathered portions an absence throughout of the colors due to iron oxide, and a variable presence 

 of carbon, giving grays to black, the inference is that the formation accumulated under a con- 

 tinuously rainy climate or one which in the drier season was sufficiently cool or cold to prevent 

 noteworthy evaporation; such climates as exist in Ireland, Iceland, or western Alaska" (294). 



On the other hand, the red colors in stratified rocks are in general due to arid and warm 

 conditions. 



"Turning to the climatic significance of red, it would therefore appear both from theoretical 

 considerations and geological observations that the chief condition for the formation of red shales 

 and sandstones is merely the alternation of seasons of warmth and dryness with seasons of flood, 

 by means of which hydration, but especially oxidation of the ferruginous material in the flood- 

 plain deposits is accomplished. * * * The annual wetting, drying, and oxidation not only 

 decompose the original iron minerals, but completely remove all traces of carbon. If this con- 

 clusion be correct, red shales or sandstones, as distinct from red mud and sand, may originate 

 under intermittently rainy, subarid, or arid climates without any close relation to temperature 

 and typically as fluvial and pluvial deposits upon the land, though to a limited extent as fiuviatile 

 sediments coming to rest upon the bottom of the shallow sea. The origin of such sediment is 

 most favored by climates which are hot and alternately wet and dry as opposed to climates which 

 are either constantly cool or constantly wet or constantly dry" (292-3). 



Red sandstones and sandy shales recur at many horizons in the American Paleozoic 

 strata and markedly so at the close of the Ordovicic, Siluric, Devonic, Lower and Upper 

 Carbonic, and early Permic. The eastern Triassic beds, and those of the Rocky Mountains, 

 are nearly everywhere red throughout, and there is considerable red color in the Lower 

 Cretacic (Morrison and Kootenay) of the Great Plains area. Then, too, there are many 

 red beds in the Proterozoic of America as well as of Europe. Between these zones of 

 brilliant strata are the far more widely distributed ones of grays and darker colors, and 



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