THE OCEAN AND ITS WORK: 



249 



deposition of some mineral matter about the grains. Other sedi- 

 mentary rocks are also compacted in the same way. 



Effect of Heat. — Sediments usually become more compact when 

 subjected to heat, as will be seen in a later discussion (Metamor- 

 phism, p. 341). 



Classification of Sedimentary Rocks 



Limestones. — The rocks of this class are composed either of 

 calcium carbonate (CaCOs) or of calcium and magnesium carbonate 

 (CaC0 3 «MgC0 3 ), and are formed in one of the following ways : (1) as 

 a result of chemical precipitation, (2) by the accumulation of the 

 calcareous coverings or skeletons of animals and plants, and (3) from 

 fragments of limestone which have been re-cemented into a solid mass. 

 Chalk (p. 523) is a limestone composed, for the most part, of the 

 remains of microscopic animals (Foraminifera). Oolitic limestone 

 consists of minute spherical concretions having much the appearance 

 of a fish roe, hence the name oolitic (p. 78). When the little con- 

 cretions are broken open and examined with a microscope a grain 

 of sand is often found in the center, around which concentric 

 layers of calcium carbonate have been added. Oolitic limestone is 

 widely used for building purposes in the United States, England, and 

 France. Travertine is deposited from solution near places where 

 springs laden with calcium carbonate emerge. In the Dinaric 

 Alps travertine forms thick beds and partially fills the basins. Such 

 deposits are not uncommon in North America in valleys of lime- 

 stone regions, where they occasionally attain a thickness of more 

 than 100 feet. Dolomite is a limestone composed of calcium and 

 magnesium carbonate in varying proportions. It is widespread and 

 often of great thickness." 



Sandstones. : — Under this term are included sandstones and 

 conglomerates. They were derived from the land, and their coarse- 

 ness or fineness (texture) depends upon their nearness to or remote- 

 ness from their source, or upon the swiftness of the currents which 

 transported them. A sandstone is composed of fine grains, while a 

 conglomerate is made up of gravel and shingle. A breccia (Italian, 

 pronounced brech'tf) is a rock composed of angular fragments larger 

 than sand, and was formed by the cementing together of the particles 

 of a much fractured or crushed rock, or of talus which had been trans- 

 ported but a short distance and therefore was not worn and rounded. 



