176 The American Geologist. September, 1903. 
The Trachytes and Rhyolytes : — They underlie the Basaltic 
sheet in many places. They also crest or flank several of the 
mountains of the region. The entire Jemez Plateau in New 
Mexico and the Apache and Pinal mountains in Arizona are 
made up of it. The Rhyolyte of the Jemez plateau does not 
overlie the Eocene (Upper Cretaceous?), but whether it was 
poured out in Cretaceous or pre-Cretaceous time, could not be 
determined by the writer. It was so long ago, however, that 
the craters and plugs have been mostly reduced to the common 
level of the country and cease to be observable at all. In the 
Pinal and Apache mountains in Arizona the Rhyolyte under- 
lies the valley fillings (Pleistocene and' Tertiary) and in the 
Hinton, (Cherry Creek) formation it both underlies the Terti- 
ary and is interstratified with it; but at no place was it found 
interstratified with or overlying the Bonneville or Quaternary 
formation. Again, there is evidence that the greater part of 
the Rhyolyte is far older than the Tertiary and Pleistocene 
overlying it. Before the deposition of the Tertiary after the 
eruption, erosive agencies reduced the lavas till only palisade 
buttes and volcanic cores remained. These project above the 
deposits to this day; and show conclusively that the sediments 
were deposited around them as they now stand ; all of the strata 
of the entire region dip in the same direction irrespective of 
their near iproximity to the volcanic cores and buttes. The Tra- 
chyte and Rhyolyte lava flows commenced in the late Creta- 
ceous and continued through the Tertiary till near the close of 
that epoch. Of the first of these flows, nothing remains except 
volcanic necks and an occasional mountain mass that has with- 
stood the degrading power of erosion, the extended sheets be- 
ing wholly removed. Of the latter flows, a great part of the 
lava still remains. The flows were often from fissure vents and 
their lavas now crest or flank many of the mountains of the 
region as we previously noticed. The outer limits of the lava 
sheet, formed at this time, has been removed and the greater 
part of the now existing sheet is overspread with Basalt where 
observed. The exception of this rule is in the Apache and 
Pinal Mountain districts, no Basalt being outpoured there. 
The Basalts : The Basalt lavas were observed to overlie 
the Pliocene and underlie the Pleistocene and Champlain form- 
ations along the Rio Grande embayment in New Mexico. They 
