The pebbles in all conglomerates
are worn and rounded by the action of water in motion, and thus
show that they have been subjected to much mechanical attrition,
whilst they have been mechanically transported for a greater
or less distance from the rock of which they originally formed
part. The analogue of the old conglomerates at the present day
is to be found in the great beds of shingle and gravel which
are formed by the action of the sea on every coast-line, and
which are composed of water-worn and well-rounded pebbles of
different sizes. A _breccia_ is a mechanically-formed rock, very
similar to a conglomerate, and consisting of larger or smaller
fragments of rock embedded in a common matrix. The fragments,
however, are in this case all more or less angular, and are not
worn or rounded. The fragments in breccias may be of large size,
or they may be comparatively small (fig. 6); and the matrix may
be composed of sand (arenaceous) or of carbonate of lime
(calcareous). In the case of an ordinary sandstone, again, we
have a rock which may be regarded as simply a very fine-grained
conglomerate or breccia, being composed of small grains of sand
(silica), sometimes rounded, sometimes more or less angular,
cemented together by some such substance as oxide of iron, silicate
of iron, or carbonate of lime. A sandstone, therefore, like a
conglomerate is a mechanically-formed rock, its component grams
being equally the result of mechanical attrition and having equally
been transported from a distance; and the same is true of the
ordinary sand of the sea-shore, which is nothing more than an
unconsolidated sandstone.
Pages:
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59