Classificaiion of Sedimentary Rocks. — Grabau. 233 
or exogcnctic rucks, still need much careful study. They are 
the rocks with which the stratigrapher is principally concerned, 
and stratigrapliic descriptions will gain much in clearness and 
precision, when stratigraphers have ceased to regard the multi- 
plication of definite and usable terms as less desirable than the 
present vague terminology. 
In hydrogenic and biogenic rocks, composition is fully as 
important as in igneous rocks, and must be used as one of the 
primary means of classification. This is not the case however 
in the exogenetic rocks, where composition is a factor rather 
controlled by accident than otherwise. Hence composition 
takes a high classificatory rank in the endogenetic rocks while it 
becomes subordinate in the exogenetic class. 
The chemical divisions of the hydrogenic and biogenic rocks 
are most satisfactorily based upon the basic elements. Only a 
few groups need to be considered as the majority of rocks fall 
under a limited number of divisions. The most important ones 
are: i. Under the Hydrogenic (a) Alkalious, or those of the 
alkaline salts, with rock salt as the most important example. 
Deposits of soda and niter when of sufficient magnitude to be 
classed as rocks, also belong here. Carnally te (KCL MgClg 
6H2O) forms a passage rock to the next group, (b) Calcareous 
and magnesian, or those of the alkaline earths. Most important 
among these are the carbonates: i.e., chemical limestones (in- 
cluding travertine, stalagmites and stalactites, etc.) and dolo- 
mytes (though many dolomytes are of secondary origin — i.e., 
the result of alteration (by diagenesis) of limestones) and the 
sulphates, gypsum and anhydrite, (c) Siliceous or the chem- 
ical deposits of silica, practically limited to the hydrous and 
anhydrous oxides of silicon, i.e. siliceous sinter or geyseryte, 
and deposits of chalcedony, jasper, vein quartz, etc. (d) Fer~ 
rnginous, represented chiefly by the oxides and the carbonate 
of iron, the latter most generally with an admixture of clastic 
material, forming clay iron stones. This latter group may 
be considered transitional to the biogenic division of rocks, 
since some limonites are of truly organic (bacterial) origin and 
since the decay of organic matter furnishes the acids which are 
active in leaching out the iron to be redeposited in a concen- 
trated state. It must, however, be borne in mind that this lat- 
ter process does not involve any physiological functions of the 
