98 REVIEWS 



The delta accumulations of a large stream like the Colorado are 

 verv different in nature of material and manner of arrangement from 

 those constituting the waste slopes or debris fans : and besides, at the 

 time the great vallevs of Arizona were filled to their present level there 

 is no reason to suppose that there was such a river as the Colorado in 

 this region. 



As a result of the attempt to discriminate the origin of the nearly 

 level surfaces of the plain-like valleys of the Southwest, the conclusion 

 was reached that those of southern Arizona, as typically represented in 

 the region about Tucson, are the result of accumulation under a body of 

 wiiter, and that this water could have been none other than the northerh 

 and eastern extension of the Gulf of California. At Tucson the plain 

 has an elevation of about 2,200 feet. 



Professor Blake has described the extension of the beds of this 

 plain into southeastern Arizona, where in the San Pedro vallev thev 

 attain an elevation of 4,000 feet. In the lower end of the vallev he 

 has found beds of diatomaceous earth which are thought to be of 

 marine origin. 



The floors of these plain-like vallevs appear dissected as thev rise 

 toward the continental divide, and upon the middle Gila river are 

 exposed in cliffs several hundred feet high and capped bv flows of 

 basalt. Basalt also forms a portion of the floor of the plain between 

 Yuma and Tucson. The beds appear nowhere to have undergone 

 other disturbance than simple uplift. Their materials are but slightly 

 consolidated, and the writer believes with Professor Blake that they 

 probably belong in the late Tertiary or early Pleistocene. 



Valleys filled with similar well-stratified material extend across the 

 continental divide, but their surfaces are in places modified by debris 

 fans and in others dissected by erosion. 



At El Paso well-stratified beds of fine detrital material have been cut 

 through by the Rio Grande. There can be no question about their 

 belonging with those farther west. They extend manv miles east of 

 the city, but just how far the writer does not know from personal 

 observation. The presumption is that thev reach the Gulf of Mexico. 



From the facts presented it is legitimate to draw the conclusion 

 that during the late Tertiary the whole of the southwestern portion of 

 the United States as well as northern Mexico was very much lower 

 than now, and that the sea reached in through long arms among the 

 mountains toward the present continental divide. Just how far it is 

 impossible to say from present knowledge, but the facts point toward 



