112 The American Geologist. August, 1905 
THE FUNDAMENTAL COMPLEX BEYOND THE SOUTHERN 
END OF THE ROCKY MOUNTAINS. 
By Charles R. Keykb, Socorro, New. Mux. 
Soon after passing the southern boundary of Colorado 
the Rocky mountains rapidly dwindle and disappear as a 
pitching anticline beneath the plains of the Mexican table- 
land. In this limited New Mexican area the Archaean, or 
Azoic, rocks form the cores of several of the principal ranges. 
The last exposure of the fundamental complex is in the 
Apache canyon, which the Atchison, Topeka and Santa Fe 
railway makes use of in crossing the mountains. South of 
this locality the only exposures of ancient crystallines are 
in the great fault-scarps of the block mountains, which rise 
out of the plains, forming the general surface of the Mexican 
tableland and the New Mexican portion of the High Plateau 
region. 
During recent years many facts have been brought to 
light which have very radically modified opinion regarding 
the great crystalline basement underlying all the Paleozoic 
sequence in New Mexico. Most of the extensive formations 
composed of granites, schists, and gneisses which form the 
axial foundations of so many of the mountain ranges of the 
region are now believed to be of much later geological age 
than is generally understood to be covered by the title, 
Azoic or Archaean. 
In summing up our knowledge on the subject, a decade 
ago. in his paper, the Pre-Cambrian Rocks of North Amer- 
ica, Van Hise* remarked : 
"It is evident from the literature that in western New Mexico 
and in the major part of Arizona is a fundamental, thoroughly crys- 
talline complex, consisting of most intricately mingled and folded 
granites, gneisses, micaceous and hornblendic schists, etc., precisely 
as in the previous sections concerned with the Rocky mountain 
system. This complex occurs at many points, constitutes the axes 
of many ranges, and its structure is of so intricate a character that 
no attempt has been made to estimate its thickness or to work out 
its structure, although in general the laminated rocks have been 
referred to as metamorphic. The granite in this complex plays the 
same part with reference to the crystalline schists as in the other 
areas referred to. Besides this ancient granite, which existed be- 
fore the next newer series of rocks was formed, there is apparently 
* Bull. U. S. Geol. Surv., No. 86, p. 331, 1892. 
