i62 The American Geologist. September, 1004. 
isolated altogether. Still beyond, the plain alone persists with- 
out notable mountains. This condition continues on the one 
hand to the gulf of California and on the other to the gulf of 
Mexico. 
At the beginning of Tertiary time the region between the 
two great gulfs north to the present Colorado line must have 
been a vast lowland plain, with but faint relief features. A 
large part of this plain was on the bevelled edges of Creta- 
ceous and older strata as is shown now in its remnants clearly 
discernible. The Las Vegas plateau, the Llano Estacado, the 
bolson plains of central New Mexico and some of the less 
broken plains of eastern Arizona seem to belong genetically 
together. To the east and west of the vast area thus outlined 
a broad submarine platform was formed from the sediments 
derived from the planing ofif of the central land area. When 
the general bowing up of the region took place later in Ter- 
tiary time the great plain formed was partly a peneplain of 
destructional land origin and partly a constructional plain of 
marine origin. 
After the period of the main uprising, after the whole sur- 
face of the country had attained somewhat more than its pres- 
ent elevation above the sea-level, normal faulting on a vast 
scale gave rise to numerous monoclinal block mountains, with 
a trend of north and south. There were numerous halts in 
the general movement and the Mesozoic and youngest Pale- 
ozoic beds here are completely stripped off the mountain sum- 
mits. Several times the staying process has enabled partial 
peneplanation to take place. But the mountain blocks have be- 
come more and more tilted. 
Between Tertiary time and the present, enormous erosion 
has taken place. The vast plain has been deeply dissected by 
such old mountain-born streams as the Canadian, Pecos. Rio 
Grande, and Colorado. The valleys of th^se water-courses are 
very wide and deep. On the east the Canadian flows 4000 
feet below the level of the old plain. The Pecos perhaps 250Q 
feet. The Rio Grande about 1500 feet. While the Colorado 
canyon is a mile deep. 
In the Llano Estacado the remnant of the great plain con- 
tains 50,000 square miles. The bolson plains are already be- 
ginning to give way to erosion agencies. In the valley of th*.* 
