Classification of ScdiHicntary Rocks. — Graban. 231 
earth magma became separated, — namely, the state of fusion, 
of aqueous sohttion, and of vapor, — and into one or more of 
which every rock mass is Hkely to return, on having its com- 
ponent materials chemically dissociated. (Compare diagram I.) 
A fourth group of endogenetic rocks which is in a man- 
ner subsidiary to the other three, is the organic or bio^^cnic 
group. The term "organic" is generally applied in a much 
more comprehensive sense, including not only those rocks di- 
rectly due to the physiological activities of organisms, as coral 
reefs, shell heaps, or coal beds, but also rocks derived from 
these by mechanical agents. As an example of these latter 
may be mentioned clastic limestones, such as make up so 
large a part of our Palaeozoic limestone beds, where the growing 
shell heaps or coral reefs are eroded and the resultant debris 
redeposited, while the reef or shell heap is still growing. It is 
true that in many cases, particularly where the uncemented 
shell heaps are worked over by the waves, undisturbed organic 
and clastic limestones may become intermingled and grade into 
each other imperceptibly. Yet if we bear in mind that classifi- 
cation should express relationship rather than differences, and 
that wherever the record is most complete in the inorganic as 
well as in the organic world, sharp lines of distinction do not 
exist, it becomes apparent that such intergradation can not be 
accepted as an argument in favor of uniting the rocks of purely 
organic origin with those derived from them while the organic 
agencies are still active. As well might we speak of the con- 
glomerates and sandstones formed from the erosion of the 
lavas of an active volcano, either by atmospheric or aqueous 
agencies, as igneous rocks, unrelated to clastic rocks formed 
from lavas of extinct volcanoes. 
Biogenic rocks fall naturally into two groups, those in 
which the material is derived from the air and those in which it 
is derived from the water. The former are due chiefly to the 
activities of plants (phytogenies and the latter chiefly to those 
of animals (Zoogcnic). Some phytogenic deposits, however, 
are derived from the water, as for example, those due to cal- 
careous algae, to diatoms, or the iron deposits due to bacteria : 
and some zoogenic rocks are derived from the atmosphere, i.e., 
deposits made by land animals. Nevertheless the most import- 
ant organic deposits from the atmosphere are phytogenic (coals) 
