The Fundamental Complex — Keyes. 117 
rocks of southwestern United States has been the compara- 
tively limited exposures. Another factor has been that the 
examination of the formations has been an incidental object 
in connection with hurried expeditions undertaken for other 
than geological purposes. In New Mexico the exposures of 
the pre-Cambrian crystalline basement are for the most part 
linear in character. There are in this region a score of 
prominent mountain ranges in which the basal crystallines 
are exposed to view. At least in half of this number the 
rocks are with but small doubt of Azoic age. Several 
ranges present crystallines which are of undoubted clastic 
origin. In the remainder the age of the crystallines is not 
definitely known. 
Most of the ranges will have to be studied anew in the 
light of the more modern conceptions rendering possible the 
differentiation of the old crystallines into well denned geo- 
logical formations. In the southern Rockies, which extend 
down from Colorado less than a third of the distance to 
the southern boundary of New Mexico, there are four large 
areas' of basal crystallines all of which, until undoubted 
elastics are discovered in them, may be considered as com- 
posed of Azoic formations. As a whole the ranges which 
collectively go to make up the southern extremity of the 
Rockies are generally known as the Snowy mountains or the 
Sangre de Cristo ranges. As the four areas of Azoic rocks 
mentioned are more or less distinctly separated from one an- 
other, they will be here taken up briefly in turn. 
The largest and most important area of ancient crystal- 
lines occurring in New Mexico is the one entering from the 
north from Colorado. Comprised within the area are the 
two important ranges, Culebra, and Taos, which are almost 
wholly made up of old crystallines. The principal rocks are 
hornblendic schists, biotitic schists, gneisses, gneissoid gran- 
ites and coarse-grained unmodified granites. Stevenson* 
frequently mentions in this and neighboring districts the 
existence of beds of quartzite in the granitic and gneissic 
rocks. Whether or not all of these "beds" are really quartz- 
itic elastics cannot now be told. From what is personally 
known of the character of the rocks generally in this region 
r. S. Geog. Sur. W. 100 Merid.. vol. iii, Supp., p. fi,8. 1881. 
