256 The American Geologist. April, 1894 
unable to cite positively am economic deposits as examples under this 
head; but there seems to be a fair probability thai such exist. 
ii. Igneous deposits of superficial origin (volcanic rocks). 
Similarly the volcanic rocks include: 
(a) Normal volcanic rocks or lavas (including tuffs), ancient and mod- 
ern. These, like the corresponding plutonic rocks, are useful chiefly 
for structural purposes; bul ii is sufficient in cite pumice in indicate 
t hat they also have other uses. 
(b) Highly differentiated and excessively basic volcanic rocks. The con- 
ditions are undoubtedly less favorable forthedevelopme.nl of the iu' us 
iron ores in lava Hows than in plutonic masses: bul this type is proposed 
in the belief thai this occurrence is nol impossible. In fact, it mighl 
a I in nst be claimed that we have aii example in the huge masses of native 
iron in t he basal 1 of ( )\ ifak. 
(c) Originah disseminations in volcanic rocks. The crystals of peridote 
(olivine) and various other gems occurring as original, and usually as 
accessory, constituents of lavas clearly justify this feature of the cla^-i 
fication. 
Each of these three types (a, l>. c) maj occur in or form — (1) contem- 
poraneous sheets or Hows: (2) volcanic cones: (3) volcanic pipes <>r 
neck-. 
id i Sublimates on recent lava, in solfataras, fumaroles, etc. The 
abundant occurrence of sulphur, boracic acid, and other minerals n\' 
economic interest in and aboul volcanic craters establishes the validity 
of t his type. 
B. Deposits of aqueo-igneous origin. — The arguments for 
the separate recognition in the lithological classification of 
the aqueo-igneous rocks or pegmatites have been stated with 
sufficient fullness. Their economic importance is manifest 
when we reflect that they are the principal source of quartz, 
feldspar and mica for commercial purposes, that they contain 
important deposits of tin ore. and that they afford tourmaline. 
beryl, and other valuahle gems. 
< '. Deposits of aqueous origin. — The aqueous deposits em- 
brace, as already explained, both those of subterranean origin 
(vein rocks, etc.) and those of superficial origin (sedimentary 
rocks, etc. ). 
i. Aqueous deposits of subterranean origin (vein rocks, 
etc.). — These have necessarily been formed by deposition 
from solution ; hut. although they are in general vein-like, 
and are as a class commonly called veins, it appears to the 
writer that in a classification based upon origin three main 
divisions should he recognized, as follows: Veins, or deposits 
in pre-existing cavities: impregnations, both metasbmatie 
