Classification of Sedimentary Rocks. — Grabau. 229 
the classification is based on the agents to which the rocks owe 
their origin. 
At the outset, a very natural subdivision of rocks into two 
great groups, the primitive and the derived or secondary,* sug- 
gests itself. Such a division would be strictly genetic and 
therefore desirable, were it not for the practical difficulty or im- 
possibility of distinguishing secondary or derived chemical, 
from primitive rocks. Thus a granite formed by cooling from 
the primitive earth-magma would in itself be indistinguishable 
from the product of cooling after refusion. In like manner a 
gypsum or salt bed deposited from the primal ocean, would 
probably differ in no appreciable character from similar beds 
derived from solution and redeposition of these older beds. The 
same thing applies to rocks of organic origin, where solution or 
decay returns the material of the rock to the water or the atmos- 
phere, from which it may be again abstracted and deposited 
by the organic agencies. 
It is then apparent that secondary rocks of these types 
must for practical reasons be classed with primary rocks of the 
same groups, whereas mechanically formed rocks may be read- 
ily recognized as derivatives. This suggests the view above ad- 
vanced, namely that the sub-division of the rocks most applic- 
able from a practical point of view, is that which emphasizes the 
agent active in their production. Thus w'e may distinguish a 
group which owes its origin chiefly to chemical agents or agents 
acting from within, or so intimately associated with the form- 
ing rock mass that the process of formation may be called 
endogenetic. Endogenetic rocks may also be called non- 
clastic, since they are never composed of fragments of older 
rocks, as are the clastic rocks, though they include regenerated 
rocks, or those in which the material of the older rocks has 
gone back to the original state of fusion or solution, from which 
it is then redeposited in a chemical way. Contrasted with this 
group is that of the clastic rocks, or those made up of the 
fragments of older rocks. These owe their origin chiefly to 
agents acting from without, and hence may be termed exo- 
genetic. These agents are chiefly mechanical, though in the 
formation of soil by atmospheric disintegration, we have chem- 
ical activities engaged in the removal of soluble constituents. 
• The Protogenoas and Denterogenons ol Nauman in part. 
