Art. IV.] Merrick, Geology of New Mexico, 85 
fragments in the hills to the south of the canon formed by it. 
In the amphitheatre-shaped cavity on the southern slope of the 
cone there is exposed a portion of a granite lying quite near 
the heart of original eruption, while above it are the brecciated 
fragments of the explosive phase of the eruption. The lime- 
stone dips into the crater cone at an angle of over 30 degrees 
and a part of its upper surface was left bare by the out-burst. 
Farther north, however, we find a spot where the original flow 
spread out over the lime. Here, in fact, was the theatre of a 
curious activity. It appears from examination of the fragments 
contained in this brecciated flow that the lava was essentially an 
andesite but where it came in contact with the lime a double 
interchange was affected. In spite of its great infusibility, the 
upper part of the lime was crystallized, forming a bed of calcite 
with cherty lammellae. On the other hand, the igneous rock 
was minutely subdivided and the fragments included in calcite 
from the lime. The resulting rock almost defies classification. 
The fragments which may be single crystals of the original rock 
or small fragments with numerous crystals, are greatly corroded 
also. About three hundred feet of this tufl* overly the lime- 
stone. So far as appears from the east, the next step might have 
been the final one in the active history of the volcano, for the 
upper three to four hundred feet up to the cones and craters on 
top is covered with a vast sheet of rhyolite with free quartz which 
is more or less plainly visible to the eye. Passing now to the sum- 
mit, the attention is held by a remarkable panorama. Evidently 
the axis of eruption rather closely correspond with the line of 
fault and so the declivity is very steep — in places almost sheer 
several hundred feet to a narrow valley which plainly displays on 
the opposite slope the up-tilted edge of the limestone in a fault 
entirely similar to the one which we have just past. Above it 
still to the westward is a series of hills of rhyolite springing 
from the fracture on the down throw side. But as we descend 
the ridge we discover much of buried history. The rhyolite 
which was the last phase of eruption did not follow directly the 
explosive stage represented by the andesite tuff but, probably 
after a period of repose, there had been more than one outflow 
