138 The American Geologist. September, 1896 
scoria-crag, from six to eight feet thick, but evidently thin- 
est near the margins. The matrix of the crag is a sort of tiitf 
with more or less concretionary^ aggregates of siliceous sand, 
but the bulk of the formation is composed of irregular frag- 
ments of scoria resembling pumice, varying in size from mi- 
nute particles to fragments eight to ten inches in diameter. 
This scoria has been quarried and used for fire kindlers, for 
which purpose it is well adapted, as it will absorb a sufficient 
quantity of oil to kindle a coal fire easily. In some places the 
crag is chiefly composed simply of concretionary aggregates, 
composed of fine particles, evidently derived from the scoria 
by abrasion. Immediately below this scoria-crag is a layer of 
three to four feet of homogeneous, rather close-grained 
"tripoli," which has been worked to some extent as a polish- 
ing powder. It is white in color and under the microscope 
reveals small flakes of the concentric plates, similar to those 
formed by powdering the scoria. These plates and scales are 
of amorphous silica glass plainly derived from the scoria. 
That the composition of the scales differs from that of a typ- 
ical acid obsidian seems to be clear from a comparison of 
analyses. The change must be due to a process of leaching 
which has increased the silica content. An analysis of the 
scoria made by Mr. S. J. Gormley, of the School of Mines of 
Socorro, is as follows: 
SiO.^ 91.206 
AI2O.; 1.022 
Fe.,o!j 634.3 
Cab .' 1.235 
MgO Trace. 
Volatile 2.14 
97.741 
An analysis of the "tripoli" immediately beneath the scoria- 
crag is as follows : 
SiO.^ 82.056 
Al.,6.j 3.746 
Fe^Oj 3.24 
CaO 911 
MgO 1.375 
Volatile 6.42 
97.748 
If our supposition is correct, that the "tripoli" is derived 
from the scoria, it is obvious that other materials have entered 
