314 PAST CLIMATES AND CLIMAXES. 



no weak link, as is perhaps best shown by its recent rapid acceptance. It 

 consists chiefly of the typical product of glaciation, namely, the ground 

 moraine or till, called tillite when ancient and fossil. The tillites are primarily 

 beds of conglomerate, the boulders of which often show characteristic stria- 

 tions. Moreover, the substratum upon which these boulder-beds rest is 

 frequently grooved and polished in the manner typical of rock strata beneath 

 present and Pleistocene glacial deposits. Such records of glaciation have now 

 been found in so many different parts of the world for several periods of Pro- 

 terozoic and Paleozoic time that they seem to place the recurrence of glacial 

 cUmates beyond reasonable doubt. 



Salt and gypsum. — Salt and gypsum beds usually occur in more or less close 

 association. They have been found in the Ordovician, Silurian, Mississippian, 

 Permian, Triassic, and Pliocene. The origin and significance of such beds 

 are thus siunmarized by Chamberlin and Salisbury (1 : 376) : 



"Gypsum appears to be deposited in quantity only in the closed basins of 

 arid regions where concentration reaches an advanced state. Since normal 

 sea-water is far from saturation with common salt, the latter is precipitated 

 only in lagoons, closed seas, or other situations favorable to great concentration. 

 This is usually achieved only in notably arid regions, and in basins that receive 

 little or no drainage from the land. Deposits of salt usually, therefore, 

 signify highly arid conditions, and where they occur over wide ranges in 

 latitude and longitude, as in certain periods of the past, unusual aridity is 

 inferred. Where confined to limited areas, their climatic significance is less, 

 for topographic conditions may determine local aridity. The total area where 

 salt is now beiag precipitated is small, though on the whole the present is 

 probably to be regarded as a rather arid period of the earth's history. On the 

 other hand, ancient deposits of salt preserved in the sedimentary strata show 

 that the area of salt deposition has been much more considerable than now, 

 at one time and another in the earth's history. The salt and gypsum deposits 

 of the past seem, therefore, to tell an interesting tale of the climates of the past." 



Red beds. — Schuchert (1914 : 273) has given an excellent summary of the 

 present interpretation of red strata: 



"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 alterna- 

 tion 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 conclusion 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 liinited 

 extent as fiuviatile sediments coming to rest upon the bottom of a shallow 

 sea. The origin of such sediment is most favored by climates which are hot 

 and alternately wet and dry as opposed to chmates which are either constantly 

 cool or constantly wet or constantly dry' (Barrell^ 1908 : 292). 



"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, 



