.384 GEORGE B. RICHARDSON 



tion. W. Spring' recently has shown that the presence of a 

 salt in water produces on a hydrate an effect comparable to that 

 of an elevation of temperature, and on this fact as a basis 

 he would account for the color of red beds. Accepting the 

 theory that red rocks are formed by the precipitation from 

 solution of ferric hydrate about the individual particles of a 

 deposit in an area of sedimentation, Spring maintains that red 

 beds were formed in estuaries or in saline lakes, where the 

 presence of dissolved salts would bring about the dehydration of 

 the precipitated ferric hydrate necessary to produce the red 

 pigment. The dehydrating effect of salt water is an important 

 contribution, but whether the pigment of red beds was deposited 

 from solution or as mechanical detritus is an independent sub- 

 ject, and one which must be settled by the study of any particu- 

 lar red formation. 



Applicatiofi of theories. — Let us now examine the evidence 

 presented by the red beds of the Black hills, in connection with 

 the requirements of these theories. 



The first, providing for the deposition of the coloring matter 

 by precipitation from percolating water subsequent to the 

 formation of the rock, clearly is inapplicable. There are no 

 available rocks which could supply sufficient iron in solution, 

 neither is there any apparent reason why the pigment, if thus 

 deposited, was limited to its present extent. Moreover, the 

 uniform distribution of the coloring matter throughout the red 

 beds in minute quantity, instead of in irregular or local accumu- 

 lations, is difficult to explain by this theory. The coating of 

 iron oxide about grains of quartz offers no suggestion that the 

 pigment is a product of replacement. And the great extent of 

 these red rocks seems to preclude the subsequent origin of 

 the pigment. 



The second theory — that the coating of pigment about the 

 rock particles was precipitated from solution during sedimenta- 

 tion — is that which has been most generally appealed to in 

 explanation of the color of red beds. The facts that such an 



'W. Spring, Recueil des travaux chimiques des Pays-Bas et de la Belgique, 

 Vol. XVII, No. 2 (1898), p. 202. 



