MAY, 1904. GEOLOGY OF WESTERN MEXICO—FARRINGTON. 219 
ern part of the hill, a black basalt, an obsidian and a diabase forming 
a hill to the north. The exact localities of these rocks are not given. 
By Ordofiez, the rocks of the Cerro Mercado are described, aside 
from the tuffs, under the general term of rhyolites. Their color is 
said to vary from gray to red and violet. The ground mass is de- 
scribed as being hard and compact, and often more or less devitrified. 
At times phenocrysts are said to be lacking, giving the rock a more or 
less felsitic appearance, and again to be so abundant as to produce 
porphyries. The phenocrysts are described as quartz and feldspar 
with some crystals of the latter reaching a length of one centimeter. 
Plates of tridymite are noted lining small cavities. The feldspar is 
stated to be partly sanidine and partly oligoclase. Witherbee gives 
two analyses of “‘the country rock, ‘cantera,’ or rhyolite,” and two of 
“white clay,’’ from the mountain.* The analyses are incomplete, 
however, and are obviously intended to be merely suggestive. More- 
over, no exact statement is made-as to the localities from which they 
were obtained. 
While no detailed study of the associated rocks of, the Cerro 
Mercado was possible to the writer upon the ground, owing to 
lack of time, hand specimens were collected at several different points, 
of which it has been possible to make careful study in the labora- 
tory. These rocks include four breccias, collected at different points 
along the northern side of the mountain, and two porphyries, ob- 
tained on the southern side. The localities where they were collected 
are shown upon the contour map, Plate LXVI. These specimens 
probably represent pretty fairly the important varieties occurring 
upon the mountain and in its vicinity. The breccias, or tuffs, such 
as were collected, characterize the whole northern side of the moun- 
tain, and extend eastward for fully half a mile. They vary con- 
stantly in colortand compactness, their general color being red, but 
at times violet and gray occur. These differences doubtless mark 
variations in the degree of oxidation of the iron, for if a portion of 
a red fragment be heated in the reducing flame its color becomes 
violet or gray. At one point in the area, evidently as the result of 
leaching, the rock is chalk-white in color. Other evidences that the 
area has been considerably permeated by circulating water are seen 
in seams and patches of chalcedony, or milky quartz, which appear 
in numerous places. This chalcedony is generally pale in color, and 
translucent. 
Of the above breccias, that collected farthest west (Mus. No. 
E 8184), E, Plate LXVI., is dull brick-red in color with lighter 
*Mining World, March 12, 1904, p. 21. 
