THE FOSSILIFEROUS ROCKS. I9 
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 dveccia 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 (aren- 
aceous) or of carbonate of 
lime (calcareous). In the case 
ofan ordinarysandstone, again, 
we have a rock which may be 
regarded as simply a very fine- 
grained conglomerate or brec- 
cia, being composed of small 
grains of sand (silica), some- 
timesrounded, sometimes more 
or less angular, cemented to- 
gether by some such substance 
as oxide of iron, silicate of 
iron, or carbonate of lime. A RES: 
sandstone, therefore, like a Fig. 6.—Microscopic section of a calcare- 
eouglénicratey’ is -a-mechaini-’ guy beoudin tes Lowe Silman (Coniston 
cally-formed rock, its Compo- The fragments are all of small size, and 
: : consist of angular pieces of transparent 
nent grains being equally the quartz, volcanic ashes, and limestone em- 
result of mechanical attrition bedded ina matrix of crystalline limestone. 
; (Original. ) ' 
and having equally been trans- 
ported from a distance; and the same is true of the ordinary 
sand of the sea-shore, which is nothing more than an uncon- 
solidated sandstone. Other so-called sands and sandstones, 
though equally mechanical in their origin, are truly calcareous 
in their nature, and are more or less entirely composed of 
carbonate of lime. Of this kind are the shell-sand so com- 
mon on our coasts, and the coral-sand which is so largely 
formed in the neighbourhood of coral-reefs. In these cases 
the rock is composed of fragments of the skeletons of shell- 
fish, and numerous other marine animals, together, in many 
instances, with the remains of certain sea-weeds (Corallines, 
Nullipores, &c.) which are endowed with the power of secret- 
