620 PRINCIPLES OF STRATIGRAPHY 



the drying westerlies, leaving the arid Coahuila basin north of the 

 delta, the central portion of which is 300 feet below sea-level ; 

 while southward the delta enters the present head of the California 

 Gulf. Occasionally a northwest distributary of the Colorado, the 

 New River, carries water into the basin, which in the past has 

 filled to overflowing, a fresh water lake being thus formed. The dry- 

 ing up of this left saline deposits upon the lacustrine beds, and 

 these, together with the eolian and occasional river deposits, form a 

 complicated series of sediments succeeding the former marine de- 

 posits of the region, all of these changes being brought about 

 without any change in the sea-level. A moderate subsidence or a 

 partial destruction of the delta would let the sea in again, and thus 

 marine deposits would once more succeed the complicated terres- 

 trial series. The area thus affected is somewhere in the neighbor- 

 hood of 5,000 square miles. (Fig. 69, Chapter IX.) 



COLORS OF CONTINENTAL CLASTICS. 



The color of clastic rocks depends to a very large degree upon 

 the states of oxidation of its iron content, and the absence 

 or abundance of carbon. A low state of oxidation gives colors 

 ranging from green to blue, while the higher states of oxidation 

 are marked by yellow, orange, brown, or red colors. According 

 to the carbon content, the color may vary from white, when no 

 carbon is present, through various shades of gray to black. Abun- 

 dant carbon in the strata will prevent oxidation of the iron and 

 will reduce the higher oxides to the lower. Lack of carbon favors 

 high oxidation. 



Sediments deposited on land or in very shallow waters are 

 especially subject to oxidation, unless there is an abundance of 

 organic matter present to prevent such oxidation. In moist or 

 pluvial climates with a moderate amount of vegetation, the soil 

 is apt to be bluish from the prevention of a high degree of oxidation 

 by the vegetation. This is especially true where the vegetation is 

 Ultimately associated with the soil. The manner in which this is 

 accomplished on the western side of Nicaragua has already been 

 referred to (p. 36). Where black soils are formed, as on 

 swampy surfaces, especially on low alluvial plains or peneplains, 

 oxidation is likewise prevented. The same thing is true of delta 

 deposits rich in carbon. Where, however, vegetation is extremely 

 luxuriant it may prevent the saturation of the soil with moisture 

 through transpiration and likewise prevent a satisfactory commin- 



