Marcou’s Geology of North America. 333 
observes respecting the great yellow (Triassic) area on the map, 
of more than 500,000 square miles: ‘‘ We now know beyond any 
reasonable doubt that all the country from the Platte to the 
British Possessions, and from the Missouri to the Black Hills 
is occupied by Cretaceous and Tertiary rocks. And as regards 
the region from the Platte southward to the Red River, very far 
the larger part is known to be not Triassic, while it is possible 
that the Trias may occur in some parts of it.” 
_ “The surface formations of the Llano Estacado, instead of be- 
ing Jurassic, are Cretaceous; this is plain from the section of 
Pyramid Mountain, and also from numerous other facts collected 
by recent explorers. If the Jurassic rocks exist there, which I 
am inclined to believe is the case, they are, as at the Black Hills, 
an underlying and not an overlying rock.” Again, “over the re- 
gion, north of the Llano Estacado which on the map is colored as 
urassic, the Cretaceous and Tertiary probably extend ; but the 
Jurassic may be looked for over a narrow outcropping belt along 
the east side of the crest of the mountains.” These observations 
are by one who has facts as a basis for his conclusions, and who 
admits a doubt until it is fully removed by investigation. 
Tn conclusion, we would say that our reconsideration of the 
labors of Mr. Marcou in America has not raised our estimate of 
their value. We know well that if any American geologist 
mapped out strata and synchronized those of America and 
tions will seriously suffer from the judgment of our ambitious 
Rocky Mountain explorer. Finally, ra ape adh a 
, that “ Marcou’s ogy a — + hie 
J. D. DANA. 
re 
sty 
