Age of the Lavas of the Plateau Region. — Reagan. 177 
are interstratified with the Pleistocene at Vallecieto Vie jo in 
the same territory. They form a part of the conglomerate 
series that underlies the Pleistocene at Jemez, four miles south- 
west of the last named place. In Arizona, in the middle Cherry 
Creek valley and in the Tonto basin, they are interstratified 
with the Pliestocene (Quaternary) and in a few cases are seen 
to underlie it ; and in the Mogollon region, in the White River 
valleys, in the Sierra Blanco district, and on the Nantanes 
plateau they overlie the Quaternary, while in the San Fran- 
cisco volcanic district, which was not visited by the writer, 
professor Gilbert says that some of the basalts are Post-glacial 
in age. 
The basaltic flows then begun in the Tertiary and contin- 
ued to the post-glacial. 
CONCLUSION. 
It is evident that eruptive activity has occured in the Pla- 
teau region from Cretaceous to recent time and at least three 
well-defined epochs are at present recognizable and at least 
two distinct kinds of lava flows, viz : 
1. The ancient volcanic necks and laccolites bordering 
the Rio Grande embayment and extending west across the Pla- 
teau to the Salt River and Gila valleys in Arizona, begun in 
later Cretaceous time, the lava sheets of which have been re- 
moved by erosion, except in the Jemez mountain district.* 
2. The Trachyte-Rhyolyte lava flows of Tertiary times, 
which are mostly pre-pliocene in age, and which are only part- 
ly removed by erosion. 
3. The Basaltic flows and cinder cones, begun in Tertian- 
time and continued to the post-glacial epoch, the last flows of 
which still maintain their original slope and extent. 
* The Rhyolyte lavas of this district may prove to be Tertiary in age upon 
further investigation. 
