TO JUNCTION OF GRAND AND GREEN RIVERS. 113 



when quite near. No fossils were found in any part of this section, and the abun- 

 dance of selenite and the profuse efflorescence of the sulphates of alumina, magnesia, 

 &c., seem to indicate the prevalence of conditions unfavorable to life during the depo- 

 sition of these strata, as well as in the similar non-fossilliferous marls of the Trias. 

 Combining the sections observed at various points between the mouth of the Animas 

 and the Nacimiento Mountain, we have, for the aggregate thickness of the conforma- 

 ble and generally similar strata resting upon the Middle Cretaceous shales, 2,000 feet, 

 made up as follows : 



Feet. 



1. Marls and sandstone forming buttes of divide 380 



2. Marls and sandstone section at Camp 48 445 



3. Marls and sandstone section at Camp 46 674 



4. Marls and sandstone in valley of San Juan '500 



Total... 1,990 



It may be suspected, from the many alternations of similar strata in the preceding 

 sections, that there has been in some cases a repetition, but that is quite certainly not 

 the case. Although no fossils were found which might serve as criteria for the iden- 

 tification of strata, the geological exposures are so full that all the more important beds 

 may be traced without break or interruption, and the order of superposition accurately 

 determined by simple inspection of the interlocking of the different sections. In meas- 

 uring altitudes the barometer was constantly used, and the heights of inaccessible points 

 frequently obtained by triangulation. By these means, and the use of Locke's level 

 where necessary, all the generalities of the thickness of strata were determined with a 

 good degree of accuracy. 



As regards the question of the precise geological age of this great group of sand- 

 stones and marls, as I have stated in the earlier part of this chapter, we have not, as 

 yet, the means of arriving at a perfectly satisfactory conclusion. The lower part of 

 the series is, we know, Cretaceous; as it contains characteristic fossils of No. IV of Meek 

 and Hayden's Nebraska section. The most natural inference, therefore, is, that a portion 

 at least of the overlying beds represent the upper part of No, IV and No. V of the 

 Nebraska section. Since there are no fossils in the upper beds, and since they are 

 apparently conformable to the lower, and generally similar to them in lithological 

 characters, we have evidence that they were deposited while the physical conditions 

 of this portion of the continent remained essentially the same. In one sense, therefore, 

 they do all belong to one great geological epoch, and, without evidence to the con- 

 trary, should be classed together. 



This argument, however, has little force, for, as we have seen, all the sedimentary 

 strata of the Colorado plateau are conformable, and there is unusual similarity of 

 lithological character among those which we know, by their fossils, belong to different 

 formations. That none of these strata are Miocene may be safely affirmed, as previous 

 to that epoch great and radical changes were effected in the physical geography of our 

 continent, and the uniformity which had characterized the deposition of the sediments 

 forming the great central plateau was entirely broken up; the sea retreating from the 

 15 s F 



