Age of the Lavas of the Plateau Region. — Reagan. \J\ 
Francisco group of volcanoes. The total area of the lava flows 
of this region in Arizona is about 25,000 square miles, or about 
half that of New York state. 
But this superficial lava sheet is only one of the great lava 
sheets that have overspread the region. It was preceded by 
two great lava flows, the lavas of which have Deen almost ob- 
literated by erosion. The earliest of these flows is now repre- 
sented only by volcanic necks and laccolites. The next earliest 
flows were fissure and volcanic eruptions, the lavas of which 
crest or flank many of the mountains of the area. The remain- 
der of the sheet then outpoured is entirely removed. 
The lavas which were ejected in this district are (1) the 
basalts, including the sanidin-dolorite types; and (2) the tra- 
chyte-rhyolyte lavas. The former were ejected Inst; at least, 
they are the predominating lavas of the last vulcanic disturb- 
ances. The latter were the predominating lavas of the fissure 
eruptions, and most probably of the earliest eruptions also, 
though propylyte and andesyte seems to have been also ejected. 
LITERATURE. 
Dr. C. L. Herrick examined a part of the lava field included 
within the Rio Grande valley, and in speaking of its age, he 
says that the group of cones from which the lavas west of Al- 
buquerque were outpoured is only one of a series of basaltic 
post-tertiary volcanoes, which can be traced along the entire 
length of the Rio Grande in New Mexico. And furthermore, 
the age of their lavas may be placed at an earlier age than that 
in which the river gravels were deposited, that is, earlier than 
the post-glacial or Champlain epoch (Bull, of the Univ. of New 
Mexico; "Environs if Albuquerque,"' p. 38; and "Papers on 
the Geology of New Mexico," p. 94.) 
The writer found basalt as an agglomerate stratum in the 
Pleistocene at Vallecieto Vie jo. New Mexico. He also found 
while digging a well at Jemez in the same territory that the 
conglomerate stratum, underlying the Pleistocene was mostly 
composed of vesicular basalt. (February No. of American 
Geologist, for 1903, pp. '87-88. ) 
Mr. Marvine, who made a geological trip from Camp Verde 
in Arizona to the Gila river, found basaltic lava underlying the 
alluvium of the San Carlos valley (U. S. Geographical Sur- 
veys west of the 100th Meridian. Vol. Ill, p. 221 ). 
