20 The American Geologist. January, 1904. 
roneous inferences. This is particularly so since specific illus- 
trations cited are certainly not examples of the types figured. 
The block mountains of central New Mexico belong 10 the 
type in which the faulting- has taken place, for all practical pur- 
poses so far as can be observed, in a single plane, as professor 
Davis has urged. In most cases one great fault along a single 
plane adequately explains the phenomena observed. There are, 
no doubt, instances in which there are several fault-planes close 
together. In New Mexico the faulting here referred to is quite 
profound, being from 300 to 400 feet. 
The Sandia mountains, east of Albuquerque, which rise 
5000 feet above the level of the Rio Grande, present evidences 
of more than a single fault, but these, faults are not nearly 
numerous enough to be regarded as true distributive faults. 
The major fault-planes are not more than three in number ac- 
cording to present observations, and are about a mile apart. 
As for the Magdalena mountains, seventy-five miles south 
of Albuquerque, and thirty miles west of the Rio Grande, the 
conditions are identically the same as in the Sandias, except 
that the fault-scarp faces east instead of west, and there is no 
reason whatever of supposing that there is more than a single 
great drop. There are many other ranges which exhibit the 
phenomena presented by the Magdalenas. The Sierra La- 
drones, the Manzano range. Sierra Oscura, Sierra San An- 
dreas, Franklin range, Sierra de los Caballos, Fra Cristobal, 
and San Mateo mountains all show similar structures. 
As diagrams of the New Mexican block mountains and 
particularly the two ranges specifically cited, the figures given 
in plate xii,* present some impossible geologic physics. From 
these figures very wrong conclusions must be sureh' drawn 
regarding the structure of the mountain ranges in question. 
Referring to the figures, the main inference is that the country 
is traversed by numberless parallel faults, a quarter of a mile 
to a mile apart, which run in a north and south direction, and 
that occasionally there occurs a great drop which produced the 
mountain range. 
My own observation has been that the many small faults 
found on the backslopes of the Sandia and Magdalena ranges. 
for example, are not of the same category as the great fault 
* Loc. cit. 
