THE FOSSILIFEROUS ROCKS. 



19 



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 (aren- 

 aceous) 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 brec- 

 cia, being composed of small 

 grains of sand (silica), some- 

 timesrounded, sometimesmore 

 or less angular, cemented to- 

 gether by some such substance 

 as oxide of iron, silicate of 

 iron, or carbonate of lime. A 



sandstone, therefore, like a Fig. 6.— Microscopic section of a calcare- 



rnnalnrnprjil-p k ^ mprliani- ous breccia in the Lower Silurian (Coniston 



conglomerate, is a mecnani- i^^^^^^^^^^ of shap Wells, Westmoreland. 



Cally-formed rock, its COmpO- The fragments are all of small size, and 



•L • 11 ,1 consist of angular pieces of transparent 



nent grams bemg equally the quartz, volcanic ashes, and limestone em- 

 result of mechanical attrition [g^f ^j^^^")^ "''''"'' °^ crystalline limestone. 



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- 



