Economic Geological Deposits, — Crosby. 253 
Veins and dikes are newer than both walls, lava flows and 
sedimentary strata are newer, always, than one wall and older 
than the other. But, for the reasons already stated, the pri- 
mary distinctions should be genetic rather than structural, 
although here, as generally, a close correlation of genesis and 
structure is to be noted. Heat and water are the two j> - reat 
agents concerned in the modification and differentiation of the 
earth's crust; and they are generally, it' not everywhere and 
always, in cooperation, heat prevailing, as a rule, at great 
depths., and water at the surface. Thus arise the two princi- 
pal classes of rocks — igneous and aqueous. In either class 
the rocks may be deep-seated (newer than both walls), or su- 
perficial (newer than one wall), in other words, intrusive or 
contemporaneous. The intrusive igneous masses are known 
as dikes, etc., and the contemporaneous as lava-Hows; while 
the intrusive aqueous masses are known as veins, etc., and the 
contemporaneous as sedimentary strata. 
Subterranean aqueous deposition must always take place 
from solution, or in a purely chemical manner; but superfi- 
cial aqueous deposition may take place from suspension ( me- 
chanically ), or from solution (chemically), or through the 
agency of organic matter (organically); and these are un- 
doubtedly, as generally recognized, the natural subdivisions 
of the superficial aqueous rocks, standing secondary to the 
distinction of superficial and deep-seated deposits. In several 
published classifications of ore deposits, however, bhe primary 
divisions of the aqueous deposits are based upon the nature 
of the formative process, whether mechanical or chemical. In 
other words, the first question asked concerning any type is, 
was it deposited from suspension or solution '■! and not, was it 
in its origin superficial or deep-seated? The disadvantage of 
this plan becomes apparent as soon as it is extended to em- 
brace all economic deposits or to have approximately natural 
boundaries ; for it associates deposits having little in common, 
and disassociates those that are closely related. Thus, a bed 
of gypsum and a vein or impregnation, say of tin ore. are, ac- 
cording to these classifications, more nearly related, since 
both are chemical deposits, than a bed of gypsum and a bed 
of slate. This may be true from the standpoint of a chemist, 
hut it is certainly not good geology; for whether we are con- 
