20 PRINCIPLES OF PALAEONTOLOGY. 



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 mechan- 

 ically-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 (are- 

 naceous) 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 sub- 

 stance as oxide of iron, silicate 

 of iron, or carbonate of lime. 

 A sandstone, therefore, like a Fig . 6 ._ Mlcro8coplCBectiOQOf acalcare . 

 conglomerate, is a mechanic- ous breccia in the Lower Silurian (Coniston 

 . Limestone) of Snap Wells, Westmoreland. 



ally-formed rock, Its COmpO- The fragments are all of small size, and 

 nent rrain<; heino- pnnnllv flip consist of angular pieces of transparent 



e quartz, volcanic ashes, and limestone em- 

 result of mechanical attrition bedded in a 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 common 

 on our coasts, and the coral-sand which is so largely formed in 

 the neighborhood of coral-reefs. In these cases the rock is com- 

 posed 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 secreting carbonate of lime from 

 the sea-water. Lastly, in certain rocks still finer in their texture 

 than sandstones, such as the various mud-rocks and shales, we 

 can still recognize a mechanical source and origin. If slices of 



