i 7 2 The American Geologist. September, 1903 
Professor G. K. Gilbert in his report on lake Bonneville 
(House Miscellaneous Documents, 1889-90, Vol. 17, p. 336) 
says : — 
I have seen in Utah, Idaho, Nevada, California, Arizona, 
and New Mexico about two hundred fields of lava, judged by 
their color and habit to be basaltic, and as many as three hun- 
dred and forty cones of basaltic scoriae .... of the streams and 
fields, 15 per cent, are judged to be Pleistocene; of the cones 
60 per cent, the remainder are judged to be Tertiary." 
Again in speaking of the age of the lavas of the west Mr. Gil- 
bert says that the trachytes and rhyolytes rest upon and inter- 
sect in dikes, all the sedimentary rocks from the Archaean to the 
Tertiary, and they were seen to underlie none of these, also 
that at every point of contact the trachytes ancr rhyolytes un- 
derlie the Bonneville beds. He continues : "The denudation of 
the trachytes themselves has been great, so great that it is meas- 
ured by the same order of unit that is applied to the denudation 
of the sedimentary beds. Not merely is the original surface 
removed from even the most recent of the trachytes but all 
craters have been destroyed and cones of mingled tuff and 
lava, that must have assumed the most imposing proportions, 
have been reduced to the merest ruins. Great dislocation, as 
well as denudation has succeeded in places the accumulation of 
trachytes and rhyolytes, and mineral veins have been formed 
within them and along their contact planes-. Great erosion cer- 
tainly, and probably dislocation also, of early eruptions have 
been succeeded by later eruptions, and stand in proof of the 
great duration of the trachyte epoch." 
Professor R. T. Hill says : — "It is evident that eruptive 
activity has occurred in the Texas-New Mexico region from 
Cretaceous to present time, and at least three well defined 
epochs are at present recognizable, viz. : 
"1. The Austin-Del Rio system or Shumard knobs; ancient 
volcanic necks or laccolites bordering the Rio Grande embay- 
ment, begun in later Cretaceous time, the lavas of which have 
been obliterated by erosion. 
"2. Lava flows of the Raton system, which are fissure 
•eruptions of Teritary times and which are only partly removed 
by erosion. 
