Art. IV.] Herrick, Geology of New Mexico. 77 
ally under the later flows. The limestone did not extend south 
of the principal Socorro peak but is abruptly cut off by a dark 
mass of andesite. The andesite projects beneath the trachyte and 
rhyolite of the mountain and has been entered by a number of 
mining tunnels, notably that of the Merritt mine which has pro- 
duced at various times considerable silver and gold. Small rich 
bits of gold-bearing' rock are encountered from lime to time, 
though the miners have hardly yet learned that it is derived 
from the andesite contact alone. To the south the deep cleft 
known as Blue Canon has cut into the trachyte crater and in so 
doing has laid bare the andesite to a considerable extent and 
here we encounter exposures of the unaltered rock so like in its 
section to the diabase porphyrite of the lake Superior region as 
to be indistinguishable. Where ever the acid rocks have come 
in direct contact with the andesite in a state of fluidity and with 
thickness enough to permit of metamorphism there seems to 
have been some segregation of ore. In other cases, where the 
two have been left in juxtaposition and where water has access 
to the contact there has been extensive kaolinization of the an- 
desite and a certain amount of similar decomposition of the 
acid rock also. These beds of kaolin which are used in the 
clay works of Socorro are very instructive as illustrating the ef- 
fects of interaction of the two rocks even without heat as a fac- 
tor. The mines of the Socorro mountain are also cases in point 
in proof of the thesis elsewhere maintained that the segrega- 
tion of gold and silver ores is closely associated with a particu- 
lar kind of metamorphism between basic and acid rocks. 
The western boundary of the trachyte area is formed by a 
series of mountains terminating in a single abrupt conical hill of 
trachyte which we have designated as “ pyramid mountain ” 
and which is skirted on the south by the new Blue Canon road. 
To the west the whole northern part of the range is composed 
of granite or gneiss. The axis of the main ridge running east 
and west is nearly as high as Socorro mountain proper and the 
contact with the trachyte is sharp with some traces of metamor- 
phism but no definite evidence as to the sequence. It may be 
presumed that the granite outpour was cotemporaneous with 
