70 
age of the Rocky Mountains, or at least, of their core. Far- 
ther north, at Santa Fé, the Carboniferous is the oldest rock 
on the flanks of the range; and above are found the later 
formations, dipping away from the line of the mountains on 
either side, and appearing in succession as the explorer 
travels either east or west. At Los Cornudos, also, in 
western Texas, there isa large mass of crystalline rock, which 
rose as an island from the Carboniferous sea. The deposits 
of that age rest against and surround it, and have filled a 
multitude of crevices in and around its flanks. But farther 
away to the west, down on the great Colorado cafion, where 
the rocks have been cut down thousands of feet, the older 
formations are revealed, in order, with the Potsdam beds at 
the base. 
Now, it appears, there are found in the Rio Grande valley, 
the crystalline base-rocks overlaid in succession by the Pots- 
dam and Primordial, the Trenton, Hudson, Upper Silurian (?), 
and Carboniferous, just as they occur in the northern and 
eastern states. The absence of the Devonian is a remarkable 
feature, but .it may be locally paralleled at many points in 
the east. The great invasions of the sea, that laid down the 
several Palzeozoic series, varied somewhat in extent, general- 
ly lessening as time passed on, until the vast submergence of 
the Carboniferous age. The Devonian sea did not reach as 
far as some of the others, at points; and did not carry its 
deposits far enough to cover this region. 
It would seem as thouch the Organ mountains had formed 
one border, and the New York and Canadian highlands the 
other, of the great Paleozoic invasions of the sea. The 
sections on the Ohio anticlinal, and many other facts, give 
evidence that these old formations extend continuously over 
the whole vast area of the continent, though largely covered 
and concealed by deposits of later age. 
