May, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 203 
the beds downward, the term hardness being here used in the sense 
of affording relatively great resistance to weathering. This increase 
in hardness may characterize the rock itself, or may be due simply 
to a softening of the upper layers through weathering. There must 
be few inherently harder layers in the rock, since, if these existed, 
whenever they came uppermost the portions below would tend to 
suffer excavation so as to form the pinnacles and pillars character- 
izing bad land tapography. At a few points in the Ciudad de 
~Rocas there may be noted a tendency to the production of such 
forms. A combination of convex and concave forms is also quite fre- 
quent. Generally, however, convex forms predominate. On the 
whole, the Ciudad de Rocas affords an illustration of the weathering 
of a relatively homogeneous, rather highly resistant rock, little jointed, 
and largely free from the influence of talus or detritus of any sort. 
Regarding the extent of the area over which the erosion forms 
are to be seen, the writer could learn nothing definite. The ranch man- 
ager, Mr. Rich, was inclined to think from such observations as he had 
made that the area was.a considerably, elongated one in outline, and 
that in the direction of its greatest’ length, a north and south direc- 
tion, it probably extended several miles, He had never explored 
the area to its limits, however, and knew no one who had. 
The rock of the area, so far as observed by the writer, was gen- 
erally homogeneous in character throughout. In color, as seen from 
a distance, it was gray. In a hand specimen, a general reddish cast 
is prominent, the color being a pale, dull red flecked with white, 
and showing violet seams. An indistinct, irregular lamination is 
visible, produced by alternate layers of red and violet, averaging 
about one millimeter in thickness. (See Fig. 3.) Occasionally the 
violet-colored seams are much wider, reaching one centimeter in 
the thickness, and probably more at times. The arrangement of 
the red and violet layers is not parallel except in a general way, nor 
is their width uniform. On the contrary, they branch and run into 
each other. Besides differing in color, the red and violet layers are 
unlike in hardness and texture. The red layers are compact, and 
have almost the hardness of quartz, while the violet layers are more 
or less pumiceous and friable in character. Cavities a centimeter 
or more in length and two or three millimeters wide are visible through 
them, and the general texture is quite open. Doubtless the abun- 
dance of these layers through the rock has aided materially in its dis- 
integration. The white flecks, previously mentioned as scattered 
through, the rock, are porphyritically developed feldspar crystals. 
They are square to rectangular in outline, and in size range from two 
