302 The American Geologist. ^^J- ^^^*- 
Los Caballos, Fra Cristobal, and tlie San Mateo mountains," as 
is done in the paper referred to. 
To take the most striking incongruity, the San Mateo range, 
as is well known, does not contain any stratified rock at all, in 
the range proper. It is a purely igneous uplift, with numerous 
flows of rhyolyte, rhyolyte breccia, tuffs, and, very likely, a core 
of andesyte or other basic eruptive. The writer has traversed 
the range on foot and on horseback in all directions and has 
never seen any stratified rock (except tuffs) in the range. There 
exist almost as great discrepancies in the remainder of the list 
given. In fact, the local differences are so great and the com- 
plications so unexpected, that it is entirely unsafe to generalize 
as to the several ranges or groups without some familiarity 
with them at close quarters. 
The Sierra Ladrones, for example, differs from the others 
mentioned in that its entire body is of granite and the granitic 
peaks form irregular pinnacles and projections completely free 
from stratified rock, which latter (Carboniferous limestones on 
the west slope) lap upon the base for a short distance in a 
broad, even curve, with none of the abrupt block faulting so 
characteristic of the Magdalenas. It certainly cannot be brought 
into the category of "block mountains." This range is, through- 
out most of its mass, more simple than most New Mexico 
mountains in that it lacks to a large extent the true later in- 
trusives. 
In the classification tentatively employed by the writer in his 
private work so far, the descriptive terms are compounded of 
two elements, the first being descriptive of the materials (I. 
plutonic, II, neptunic, III, metamorphic) the second descriptive 
of the origin (I, erosive, II, eruptive, III, warping, IV, plastic- 
readjustive — plastic for brevity), and any range may be simple, 
compound or complex, or may be compound and complex. 
In this classification, using the complete descriptive phrase- 
ology, the Magdalenas would be compound, complex, plutonic- 
metamorphic-neptunic, eruptive-plastic-erosive (and probably 
warping ought to be added). Warping commonly is involved 
with the plastic condition. The San Mateos, on the other 
hand, would be simple, plutonic, eruptive-erosive. The La- 
drones are simple, neptunic-metamorphic, plastic-erosive (the 
main bodv of the mountain is here under consideration). 
