290 STUDIES FOR STUDENTS 



diffiisable, permeating the entire rock mass and thereby becoming 

 more effective as a coloring substance. In view of this ready diffusa- 

 bility and ease of dehydration, such a special hypothesis as that of 

 Russell — that the crusts of ferric oxide had been retained by sand 

 grains during their transportation from a residual soil — seems un- 

 necessary and as a general explanation does not apply. 



To sum up, it is seen that the cause of the red color in ferruginous 

 rocks as contrasted with the predominant yellows of modern alluvium 

 is to be found in three co-operating causes : First, spontaneous dehy- 

 dration operates to some extent at the surface in the warmer regions. 

 Second, dehydration under great pressure and moderate temperatures 

 is nearly universal in sediments which become buried and consolidated. 

 Third, dift'usion operates under conditions of warmth and moisture, 

 whether these be found at the surface, as in warm and humid regions, 

 or beneath the surface, as may occur in any portion of the earth. By 

 these three means light-colored, yellow or brown muds and sands 

 may become red shales and sandstones. Only in the presence of 

 considerable heat, as on the walls of dikes, or in the presence of some 

 highly dissolving fluid does the tendency toward crystallization or new 

 combination reverse the coloring effects of capillary diffusion. 



Red in shales or sandstones is therefore normally assumed, like 

 the hardness, upon the consolidation into a shale or sandstone of any 

 sediment possessing an appreciable amount of ferric hydrate and is no 

 more necessarily original than is the red of a burned brick. 



The reliability of these conclusions may be tested by observing 

 the stratigraphic relations of ancient deposits. As a typical example 

 may be cited the Permian red-beds developed east of the Rocky Moun- 

 tains, which contain conspicuous strata of gypsum and are impreg- 

 nated with salt, giving thus undoubted evidence of deposition under 

 an arid climate. The same association of salt and gypsum with red 

 shales and sandstones might be cited from Nova Scotia and a dozen 

 other localities.. 



This is in striking contrast to the usual present development of 

 salt and gypsum in association with gray or yellow sediments. For 

 example, the marginal bottom of the Dead Sea when exposed by 

 unusual dessication shows a surface of bluish-gray clay or marl full 

 of crystals of common salt and gypsum, and light-grays are char- 



