230 Tlie American Geologist. April, 1904. 
and the alteration of others. Nevertheless the agents active 
here are external, and the resultant detrital mass or residual 
soil must be included with the clastic or exogenetic rocks. 
Returning now to the endogenetic or chemically deposited 
rocks, we may readily distinguish four groups, which, though of 
unequal quantitative value, with reference to their distribution 
in the earth's crust, are nevertheless of similar taxonomic im- 
portance. The first of these groups includes the well recog- 
nized Igneous rocks, to which the term pyrogenic is applicable. 
Kemp has clearly shown* that ice is to be classed as an ultra 
basic igneous rock, since water has the essential characteristics 
of a rock magma, though it is much less complex in composi- 
tioUj and is able to remain unsolidified at a much lower temper- 
ature than other magmas. This applies, however, only to 
water-ice. Snow-ice on the other hand is a truely sedimentary 
rock, which is separated from. the atmosphere as snow, much 
as crystals of salt or gypsum are separated from water. The 
importance of snow and snow-ice as a rock can hardly be ques- 
tioned, and it is also readily seen that they must be placed in a 
division by themselves, namely, the second, or atmospheric di- 
vision, to which the name atinogenic is applicable. 
The third great group of the endogenetic rocks comprises 
those ordinarily designated the chemical rocks, though it is 
manifestly incorrect to limit the term chemical to those rocks 
deposited from solution in water. But the term aqueous or 
Jiydrogenic is applicable to them and should be restricted to 
this group. It seems to me that the term "aqueous'' is much 
less applicable to the clastic rocks generally included under this 
head, for in them water is only the external agent, as heat 
is in the pyro-clastics, which are not generally included with the 
igneous rocks. Thus the restriction of the term aqueous to the 
aqueo-chemical rocks, (hydrogenic) and the separation from 
them of the aqueo-mechanical rocks under the term hydro- 
clastic^ is in perfect accord with the practice of separating the 
igneo-chemical rocks under the term igneous (pyrogenic) from 
the igneo-mechanical rocks under the term pyroclastics. 
These three groups, the pyrogenic, atmogenic, and hydro- 
genic, represent direct deposition, in a chemical manner, of ma- 
terials from the three several states into which the primitive 
* Handbook of Rocks. 
