Economic Geological Deposits. — Crosby 255 
abrasive, refractory and other qualities, ;i wide range of applications in 
the arts. 
(b) Highly differentiated <iu<l excessively basic plutonic. rocks. These are 
peridotites and other wit ra-basic nicks, in which, through t In- ope rat ion. 
we may suppose, of Soret's principle, as [ddings insists, aided, possibly. 
by magnetism and gravitation, as suggested by Vogt, tin' metallic ox- 
ides or free bases (magnetite, menaccanite, etc.), which occur only as 
relatively unimportant accessories in the normal types, become highly 
concentrated and give character to t lie rock, constituting a very distincl 
and not unimportant type of iron ore. The dike or stock of titanifer- 
ous magnetite, with accessory olivine and feldspar, of Cumberland, K. 1.. 
is a good example: and these ores have also been recognized in the Adi- 
rondacks, Minnesota. Brazil, and Scandinavia. It appeal's, in fact, that 
lite highly titaniferous, crystalline iron ores should be. in general, refer- 
able to this type. 
(c) Original disseminations in plutonic rocks: Not the rock as a whole. 
but a single original constituent, is useful. The useful mineral may be 
accessory, as the zircon in certain syenites: or essential, as the chatoy- 
ant labradorite of certain norites and gabbros. This type is not mark- 
edly differenl from the preceding (b). Only one constituent or class of 
constituents is. strictly speaking, useful in each case; but the magmatic 
concentration is an essential feature of (b) and the entire rock or ore 
must be mined and submitted to metallurgical treatment. The last 
consideration savors somewhat of a classification by uses, and the chief 
emphasis should, therefore, be laid upon the concent rat ion of (b) as the 
more essential distinction. 
Each of these types (a, b. <•) may theoretically, at least, occur in or 
form — (1) dikes, (2) intrusive sheets or sills, ('.',) laccolites, (4) stocks or 
plugs, bosses and complexes. 
(d) Sublimates formed in dry fissures or other cavities, and not neces- 
sarily in plutonic rocks; although heat, of course, is still the chief agent. 
This type — virtually veins formed by sublimation — is proposed with hes- 
itation. Sublimation is at the present lime, and justly, in marked dis- 
favor as a general explanation of mineral veins. Below the permanent 
water-level the vacant fissures essential to the formation of sublimates 
must be non-existent; and subterranean sublimation deposits, if occur- 
ring at all, must be limited in depth; while the com posit ion and structure 
of most veins are best explained by deposition from solution. On the 
other hand, the varied and abundant sublimates formed on the surfaces 
and in the cracks of recent lavas make it more than probable that, at 
least in the neighborhood of volcanic vents, they extend to a sufficient 
depth to deserve classificat ion among subterranean deposits: ami we ma.v 
safely conclude that sublimation is a true, though not a principal, cause 
of vein-formation. This conclusion is sustained bv the fact thai a 
large proportion of the vein-forming minerals have been produced ex- 
perimentally by sublimation. Although accepting the sublimation 
theory as a valid explanation within the narrow limits indicated. | am 
