Age of the Lavas of the Plateau Region. — Reagan. 175 
g. Red shale, graduating below to soft red sandstone 240 
h. Red and gray shale interlaminated with gypsum 70 
i. Calcareous sandstone with prints of sun-cracks 1 
j. Red, shaly, ripple-marked sandstone, with red-clay shale and 
red sandy shale 120 
k. Unseen, shale 50 
* Total 2260 
STRATIGRAPHY. 
The Carboniferous rocks, wherever exposed, are very eas- 
ily identified by their fossils : Productus cora, P. costatns, .P 
Nebrascienses, Spirifer cameratus, etc., but the rocks overlying 
the Carboniferous and non-conformable with it in the Plateau 
region are usually non-fossiliferous and are not easily identi- 
fied. The upper part of the last named rocks, on account of its 
loose unlithified condition, is readily determined to be Quater- 
nary ; but the age of the rocks below the middle of the series 
is unknown, except that their upper part is late Tertiary. Mr. 
Gilbert, in correlating his stratigraphical sections, one of which 
is given here, places the unconsolidated sands and clays in the 
Quaternary at the top, but leaves the age of their lower por- 
tions undermined. He says : "They are of Quaternary age 
at top, but the antiquity of their lower portions is unknown. 
It is not improbable that they, in some valleys, accumulated 
through all Tertiary time, and they may have been begun even 
earlier." (U. S. Geological Surveys, west of the 100th Merid- 
'an, Vol. Ill, p. 172.) 
That the lower portion of the series is Tertiary at top, there 
is plenty of stratigraphical evidence. The Sombrero (Hinton) 
deposit on Salt river in Arizona, is capped with a cobblestone 
conglomerate stratum, identical with that which caps the Santa 
Fe Marls of Cope at Algodones, New Mexico ; and below this 
cap the strata are. partly lithified sands and clays, the same as 
the strata of the marls just mentioned. But how much of this 
formation is Pliocene and how much is earlier Tertiary was not 
determined. 
AGE OF THE LAVAS. 
In considering the age of the lavas, the Trachytes and Rhy- 
olytes will be considered first, the Basalts last. 
•The age of the rocks given above was inserted by the writer. 
