MECHANICAL DEPOSITION. 87 



of certain streams and at the mouths of some rivers, are thrown down. 

 In ancient times also, the most abundant chemical deposit from water 

 was limestone. 



The chemical stratified deposits are principally limestones, or com- 

 posed of carbonates of lime and magnesia, or are salt rocks with beds 

 of chloride of sodium. This is not the place to discuss points of 

 theory, and we shall therefore speculate no further at present on the 

 origin of these deposits than to say, that the quantity of lime now 

 held in solution in sea-water is subject to daily diminution through 

 the agency of life, and experiences daily renewal by the inflow of 

 streams from the land. The innumerable tribes of corals, mollusca, 

 and other invertebrata, obtain the carbonate and phosphate of lime 

 necessary for their skeletons, &c., from the salts of lime in the sea, 

 and these salts are supplied by streams from the land, which have 

 derived lime from the old rocks. The calcareous rocks are found to 

 be almost wholly composed of shells, corals, Crustacea, &c., and thus 

 we perceive as a very general fact, that it is less by direct chemical 

 reactions than by vital energy and the decay of organised fabrics, 

 that thick calcareous masses of every geological age have been formed 

 and are still forming in the sea. 



Mechanical Deposits. The mechanical agency of water is manifest 

 in removing materials from one place and depositing them in another. 

 Thus pebbles and sand and clay are transported by the tides and by 

 rivers, and accumulated in low situations in regular layers, miniature 

 representations of those thicker strata of the same ingredients which 

 compose the crust of the earth. And as at the present day some 

 materials are transported farther by water than others, and consequently 

 more rounded by attrition, so the materials of the strata are likewise 

 more or less worn and rounded, in proportion to the distance they 

 have travelled and the friction they have suffered. 



In many situations chemical and mechanical products are thrown 

 down successively by the same waters, just as in the older strata 

 limestones and sandstones occur alternately. We see, therefore, that 

 the ancient deposits from water, which form layers several miles thick 

 around a great part of the globe, are not essentially different, except in 

 degree, from the lesser deposits now formed beneath the sea, and 

 by streams from the land. 



The mechanical deposits or strata composed of earthy materials, 

 are distinguished by the coarseness, or fineness, or nature of their 

 ingredients. The following scale will convey some notion of the 

 gradations of size in the ingredients of mechanical deposits : 



Very fine particles, generally containing ) C1 j shal and slate . 



20 to 30 per cent, of alumina ] J ' 



Mixture of clay and sand Sandy clay. 



Sand with some clay Argillaceous sandstone. 



Small fragments of hard siliceous minerals. Sand, sandstone. 



Sandstone including pebbles Millstone grit. 



Large pebbles united by sandstone or clay... Conglomerate or puddingstone. 



Pebbles disunited Gravel. 



Angular stony fragments reunited Breccia. 



