i82 The American Geologist. September, i904 
instance, yet it is not common to find both sides clearly pre- 
served at any one cross-section. The eastern limb is seen in 
the Sandias and Manzanos, the western, farther south in 
the Ladronnes, Limitars and Socor/n ranges, while still 
further south the river turns west but remains in the anti- 
clinal valley, the eastern border of which is formed by the 
San Cristobal and Caballos ranges. The eastern limb is nearly 
covered east of Socorro.* 
We do not conceive of this great fold (nor that ea.st 
of it, forming the basin under consideration) as narrow plica- 
tions but rather as great undulations wi*:h irregularities which 
may in places almost obscure the general north and south 
trend. The foundation for these orographic changes was laid 
after the upheaval that closed the granite-forming period, that 
is prior to the deposition of the Carboniferous (farther south, 
Devonian) limestones that repose upon the granite. The 
upper surface of the granite is planed to such an extent that 
the contact at a distance appears a right line. These sedi- 
ments, in so far as they are not calcareous and so derived di- 
rectly from the ocean, seem to have come from the surviving 
peaks or land masses of granite not covered by the Carbonif- 
erous sea and there is every reason to believe that during the 
Carboniferous-Permian interval (if interval there was) an up- 
lift presented more of the granite material to the tooth of the 
waves. The unconformities thus indicatf^d seem to have been on 
a large scale and are so broad in their contours as to defy detec- 
tion in a small field of observation though when once worked 
out they may be strikingly obvious. Reverting to our de- 
scription, we again quote Mr. Russell : "The valleys or plains 
separatmg the mountain ranges . . . are often absolute des- 
erts, totally destitute of water, treeless for many days' jour- 
ney, the grayish green Artemisia or "sage brush" giving 
character to the landscape. Many of them have playas in 
their lowest depressions — simple mud nlains left by the evap- 
oration of former lakes. ... In the summer months portions 
of them become so baked and hardened as scarcely to receive 
an impression from a horse's hoof, and so sun-cracked as to 
resemble tessellated pavements of cream-colored marble. 
* Laws of Formation of New Mexico Mountaiu Ranges. .\mkrican 
Geologist, May, 1904. 
