SILICEOUS AND ARENACEOUS ROCKS. 
35 
CHAPTER 11. 
AQUEOUS ROCKS.*—THEIR COMPOSITION AND FORMS OF STRATI¬ 
FICATION. 
Mineral Composition of Strata.—Siliceous Rocks.—Argillaceous.—Calcare¬ 
ous.—Gypsum.—Forms of Stratification.—Original Horizontality.—Thin¬ 
ning out.—Diagonal Arrangement.—Ripple-mark. 
In pursuance of the arrangement explained in the last 
chapter, we shall begin by examining the aqueous or sedi¬ 
mentary rocks, which are lor the most part distinctly strati¬ 
fied, and contain fossils. We may first study them with 
reference to their mineral composition, external appearance, 
position, mode of origin, organic contents, and other charac¬ 
ters which belong to them as aqueous formations, independ¬ 
ently of their age, and we may afterwards consider them 
chronologically or with reference to the successive geologic¬ 
al periods when they originated. 
I have already given an outline of the data which led to 
the belief that the stratified and fossiliferous rocks were 
originally deposited under water; but, before entering into 
a more detailed investigation, it will be desirable to say 
something of the ordinary materials of which such strata 
are composed. These may be said to belong principally to 
three divisions, the siliceous, the argillaceous, and the calca¬ 
reous, which are formed respectively of flint, clay, and car¬ 
bonate of lime. Of these, the siliceous are chiefly made up 
of sand or flinty grains; the argillaceous, or clayey, of a 
mixture of siliceous matter with a certain proportion, about 
a'fourth in weight, of aluminous earth ; and, lastly, the cal¬ 
careous rocks, or limestones, of carbonic acid and lime. 
Siliceous and Arenaceous Rocks. —To speak first of the 
sandy division: beds of loose sand are frequently met with, 
of which the grains consist entirely of silex, which term 
comprehends all purely siliceous minerals, as quartz and 
common flint. Quartz is silex in its purest form. Flint 
usually contains some admixture of alumina and oxide of 
iron. The siliceous grains in sand are usually rounded, as 
if by the action of running water. Sandstone is an aggre¬ 
gate of such grains, which often cohere together without 
any visible cement, but more commonly are bound together 
