288 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
This evidence, however, is as yet difficult of interpretation, because 
the climatic factors are not easily separated from those due to topo- 
graphic 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 semiaridity to Upper 
Carboniferous coolness, humidity, and glaciation (1908). 
In regard to the significance of gray to black formations Barrell 
states: 
Where a whole formation, representing an ancient flood plain or delta, shows in its 
unweathered portions an absence throughout of the colors due to iron oxide, anda 
variable presence of carbon, giving grays to black, the inference is that the formation 
accumulated under a continuously 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. 
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 oxida- 
tion 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 tempera- 
ture and typically as fluvial and pluvial deposits upon the land, though to a limited 
extent as fluviatile 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. 
Red sandstones and sandy shales recur at many horizons in the 
American Paleozoic strata and markedly so at the close of the Ordo- 
vicic, 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 gonsiderable 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 
