The 8o-C ailed Socorro Tripoli. — Herrick. 139 
into the former — such, no doubt, as form the earthy portions 
of the scoria-crag. 
It may be noted that man}^ samples of the scoria have mi- 
nute microscopic crystals which polarize highly and have the 
optical properties of quartz and sanidin respectively, other- 
wise the siliceous lamellae are isotropous in both "tripoli" and 
scoria. 
Beneath the "tripoli" is a bed of yellowish s-ind, with irreg- 
ular layers of tough reddish clay; still below this is a very 
fine grained stratum of j^ellowish "tripoli" which, while evi- 
dently less pure, is mechanically better suited to serve as a 
fine abrasive than the relatively coarse upper layer. Its 
composition, according to the analyses made by Mr. Gormley, 
is as follows : 
SiOg 79.416 
AI2O3 1..391 
¥q.,6.^ 6.245 
Cab 2.4.3 
MgO 1.485 
Volatile 6.06 
97.04 
The silica content is here only a little higher than in some 
acid glasses, but the evidence shows that this composition is 
due to foreign admixture. 
As to the method of formation we are shut up to a narrow 
range of possibilities. It may be assumed that a local basin, 
at a comparatively late period in the history of the valley and 
during an interval of subsidence of its waters, was filled by 
debris from a scoria bed. At this time the river erosion may 
have exposed and attacked such a flow as that first above de- 
scribed, and either transported the floating scoria to the rela- 
tively quiet eddy where they triturated each other until the 
tripoli was formed or else the debris of such triturated materi- 
al was swept into the quiet retreat. In any event, with the 
filling of the basin the flotsam of scoria Anally was stranded 
in the superficial laj'^ers forming the scoria-crag as we now 
find it. The extent of the beds has been but roughly esti- 
mated at from 800 to 400 feet in east and west diameter, and 
eight to ten times as great in north and south extension. A 
large portion has been eroded and washed away by periods of 
high water, although it is now far al)ove reach of the river 
action. 
