268 The AmerxGan Geologist. May, i89o 
stream from this it has cut into the lava only five or ten feet. 
The side streams tributary to Taos creek exhibit the same 
phenomenon on a smaller scale. This shows with what ra- 
pidity the erosion goes on after the lava is cut through and 
how young the drainage really is. In these creeks we see on a 
small scale what the Rio Grande has been doing on a large 
scale since the basalt flow first interruped its course. 
The youth of the present Rio Grande drainage system is 
equally well shown in regions not covered with lava. Such 
places are also rejuvenated, in the north at least, by the lava 
flows, which, by impeding the Rio Grande itself also impeded 
all the side streams. At present these places are only just re- 
covering from the accident. If the region was a moist one there 
would be no such signs of present recovery as can be seen there. 
As it is, in this case the rainfall peculiarities aid in the ap- 
pearances of extreme youth. Aside from the main streams 
having sources in the mountains, there are few which carry 
water all the year, and not a great many Avell established trib- 
utaries to carry off the excessive floods to which the country is 
sometimes subject. Consequently every heavy rain develops 
new branches to the smaller tributaries. These "arroyas," as 
they are called, develop suddenly, even in a single storm, and 
often have a linear extension of several miles and a depth of 
fifteen to twenty-five feet with canon-like walls of gravel. 
Other arroyas are growing slowly year by year at their head- 
waters. Very frequently an arroya eats back across a well 
traveled road, sometimes suddenly in a single storm ; but very 
often slowly year by year so that each year the road is forced 
to make a detour greater and greater in extent until in some 
cases the angle in the road at the head of an arroya is as much 
as a quarter of a mile from a former position a few years pre- 
vious. 
When a small arroya is formed many tons of earth are re- 
moved and turned over to the main stream as a burden to be 
carried to the sea. While the Rio Grande was cutting its own 
canon it was able to remove the detritus, partly because it was 
not particularly overburdened and partly because its slope 
seaward Avas rapid. Now the slope is reduced while the river 
is busy removing dams at various places (as at El Paso) ; 
and the main stream is burdened with sediment furnished 
by all its tributaries which are busily at work establishing 
