Jenica — Albuquerque, N. Mex. — Reagan. 91 
ive and poured out the great rhyolitic sheet which now, on the 
plateau, covers the granite porphyritic core of the Jemez range, 
over which these same volcanoes hurled out 120 feet of . pum- 
iceous tuff. These mountains were still islands in Cretaceous 
times, but their area then w^as much greater than formerly. In 
this period the mountains seem to have been gradually rising 
until in the Fort Union epoch great swamps covered the entire 
country, the sea being obliterated for a time. In the swamps 
vegetation was luxuriant ; and the vegetable matter laid down 
in them forms today the coal fields of northwestern New Mex- 
ico. At the close of the Fort .Union epoch there was a slow 
subsidence. The Puerco was deposited on the Fort Union, 
followed by Eocene, the whole series being conformable. Then 
there came a violent change. The whole country was elevated 
above the sea; much faulted and broken up; and blocked basins 
on a grand scale resulted. The depressions were the lakes of 
Pliocene times. One large lake existed in the vicinity of 
Jemez, another, in the Rio Grande valley. The lake at Jemez 
was filled up with the Jemez marls by the tributaries of the 
Jemez river, and the Rio Grande lake was silted up with the 
Albuquerque marls, probably by the tributaries of the river 
which at present occupies that valley. When these lakes were 
nearly filled, there was a further re-elevation of the country 
and the rivers at once began to incise channels for themselves ; 
but this deepening of their channels was suddenly arrested by 
seismic disturbances and the lava flows of Post-Tertiary times. 
The former changed the incline of the river chjannels, and the 
latter damned the rivers, thus forming lakes. In these lakes 
were deposited the Pleistocene depostis of the river valleys, the 
Placita marl, the alluvium, etc. At the close of the Pleisto- 
cene, these lakes in turn were obliterated and the country took 
on the general appearance that it has today. 
Geography and Physiography. 
Rivers. — The rivers of the section are : the Rio Grande, the 
Rio Puerco, and the Jemez. 
The Rio Grande. — The Rio Grande river flows to the east 
of the Jemez mountains and to the west of the San Dia escarp- 
ment. It is west of Albuquerque and east of San Felipe. It 
is a stream an eighth of a mile in width at San Felipe ; at high 
water it is often a half a mile or more in width at the village. 
