
60 STRUCTURAL AND FIELD GEOLOGY 
case with igneous rocks, sedimentary rocks of various kinds 
often merge into one another—coarse-grained grits and sand- 
stones, for example, passing gradually into the finest argil- 
laceous accumulations. The coarser-grained accumulations 
are almost invariably of shallow water origin—deposited at 
or opposite the mouths of rivers and along the sea-shore 
between low- and high-water levels. The medium grained 
masses have been laid down generally in somewhat deeper 
water—or in places where aqueous action was less strenuous. 
The finest grained sediments have accumulated in still water, 
and therefore usually at some distance from the land. Such 
being the origin of sedimentary rocks, it is not surprising that 
they should frequently contain the relics of animals and 
plants, z.e. fossil organic remains. 
Conglomerate is a bedded or amorphous aggregate of 
waterworn stones, and may be either of marine or freshwater 
origin. It is, in short, simply a more or less consolidated 
gravel. The matrix in which the stones are set is usually 
gritty or sandy, and may be scanty or abundant. The rock 
often graduates into pebbly grit and conglomeratic sandstone. 
The cementing material may be siliceous, calcareous, argil- 
laceous, or ferruginous. Quartz and hard siliceous rocks are 
usually the most conspicuous components of conglomerate. 
Aqueous Breccia is a consolidated rock-rubble, which has been 
accumulated in water. 
Grit and Sandstone.—These are simply coarser and 
finer grained varieties of one and the same kind of rock— 
namely, compacted or cemented grit or sand. The most 
abundant component is usually quartz, but many other 
ingredients may be present. Amongst these may be felspar 
(more or less kaolinised), and occasionally some of the less 
readily decomposed minerals derived from the disintegration 
of igneous and schistose rocks, such as zircon, schorl, garnet, 
etc. The finer grains of an aqueous sandstone, unlike those 
of desert sand, are often angular or subangular, while the 
larger grains and small pebbles are usually well waterworn 
and rounded. Sandstones may be white, grey, yellow, brown, 
red, greenish, or black. The colouring matter is in most 
cases due to that of the cementing or binding material, which 
may either be dispersed between the grains, as in carbonaceous 

