Colorado River of the West. «397 
ves, with Ammonites, Gryphea, and Inoceramus of Cretaceous species. 
ese fossils leave no room for doubt in reference to the age of the strata 
which contain them, but prove them to be Lower Cretaceous.” 
The enormous thickness of strata is at places surmounted by 
another series of great thickness. This series is thus alluded 
to by Dr. N. 
“Going north from the Moquis villages, on the Lower Cretaceous mesa, 
ress was arrested by a want of water; the surface being every- 
and its larger tributaries. Before we turned back, however, we had ap- 
proached nearly to the base of a wall rising abruptly from the mesa in 
Which we stood to the height of more than 1,000 feet. is wall was as 
white as chalk, and reflected the sunlight like a bank of snow. It is 
evidently the edge of another and higher plateau, and apparently reaches 
to the Great Colorado, where it caps the ‘hig mesa,’ forming part 
the stupendous mural faces, pr | 
Were distinctly visible when we 
& hundred miles. 
What is the character of this upper mesa I had no means of deter- 
Mining at this time, and even now there may be some poe about it; 
t I have scarcely a doubt that it is composed of the | pper Cretaceous 
strata, the equivalents of the ‘ white chalk’ of Europe. 
In regard to the causes which have produced the remarkable 
topographical features of this interesting region, Dr. Newberry 
shows that it is not due, as would probably be supposed by one 
not accustomed to the study of such phenomena, to voleanic or 
eruptive agencies, but solely to the erosive action of running 
Water. 
Thus he continues: 
“The sketch which has been given of the table-lands of the upper 
Colorado, though brief, will perhaps suffice to convey an idea of the gen- 
eralities of their structure and relations. But before returning to the 
details of the local geology of our route, I ought perhaps to refer briefly 
to two questions of general import, which would naturally suggest them- 
