Drainage Systems of New Mexico. — Tarr. 265 
quantities of basalt flowed. There is very little doubt that 
most if not all the Tertiary basalt in this section can be direct- 
ly traced to some volcanic cone. 
When the basalt flowed from the various cones it formed 
dams at many places across the developing rivers. One of the 
principal of these dams was at the eastern base of the "Volca- 
no" cone and thence southward for 60 miles. The eff"ect of this 
dam was to make a large lake having an area of several hun- 
dred square miles, since drained, and now called the San Luis 
Park in southern Colorado. This rejuvenation of the young 
Rio Grande system took place at many points and numerous 
small lakes were formed principally at the mouths of west-flow- 
ing tributaries. One of these may be seen at the town of Em- 
budo near the mouth of the Embudo river. These lake de- 
posits can be distinguished from the earlier Tertiary sands by 
the great abundance of basaltic pebbles in the former. In 
some places the upper layers of the great Tertiary conglomerate 
contain occasional basalt pebbles showing that the Tertiary 
lava period began before the final drainage of some of the lakes. 
During the deposition of this lava the entire country was 
undoubtedly uplifted and when it ceased a new drainage sys- 
tem had to be established and a great work of rapid erosion to 
be begun, although the time has been shbrt, the amount of 
erosion since the end of the basalt period has been very great. 
The rejuvenated Rio Grande has in every place succeeded in 
cutting through the lava cap and in many places has eroded 
far into the soft underlying deposits. From the southern end 
of the San Luis Park to below Embudo there is a great caiion 
averaging over a thousand feet in depth with a length of more 
than 60 miles and a width ranging from two thousand feet to 
two miles. The lava capping in the northern part l)eing very 
thick, the underlying unconsolidated strata arc protected and 
the caiion character of the river valley is well preserved. At 
the southern end, however, near Embudo, where the lava is 
only about twenty feet thick the canon is broadening and 
rapidly loosing its distinctive steep-walled character. The 
wearing back of the basalt by subaerial denudation has broad- 
ened the canon to a width of two miles. The soft clays and 
sands thus left exposed arc rapidly melting away and are strewn 
with thousands of basalt boulders. Some of the hills are still 
lava-capped although they have sunk down several hundred 
