212 THE SEDIMENTARY ROCKS 



fineness of grain, and crystalline texture. Sand or clay is fre- 

 quently present as an impurity, and by an increase in these mate- 

 rials, the limestones pass gradually into sandstones and shales. 

 In some varieties of limestone the organic nature of the rock is 

 most obvious, shells, corals, crinoid stems, and the like being con- 

 spicuously shown, especially on weathered surfaces. In other 

 kinds the microscope is required to make this organic nature 

 clear ; while in others, again, the calcareous materials have been so 

 ground up by the action of the waves, that all traces of organic 

 structure have disappeared. The example of the reef rock now 

 forming in many coral reefs (p. 168) is a warning that the absence 

 of even microscopic structure in a limestone cannot be relied upon 

 as a proof that the rock is not of organic origin. 



The great limestones are almost entirely of marine origin, though 

 quite extensive fresh-water limestones are known. The chemically 

 formed ones are never very widely extended, though they may 

 form quite thick masses. As a rule, the limestones are deposited 

 in deeper water than the sandstones and shales, but not necessarily 

 so, freedom from large amounts of terrigenous sediments being 

 more important than depth of water. This is shown by the great 

 calcareous banks of the Gulf of Mexico and the Caribbean Sea 

 (p. 172), and coral reefs are always formed in shallow water of 

 less than twenty fathoms in depth. 



The classification of the limestones is very difficult, and cannot 

 be readily made on any single principle ; mode of formation, 

 purity, texture, and nature of organic material, all being employed 

 for the purpose. 



Shell Marl is an incoherent and crumbling rock, formed, prin- 

 cipally, at the bottom of fresh-water lakes and ponds, by the 

 accumulation of shells ; it frequently occurs beneath peat bogs, 

 and is an indication that the bog arose from the choking up of a 

 lake by vegetable growth. When the shells are cemented into 

 a hard rock they form a fresh-water limestone. 



Chalk is a soft limestone of friable, earthy texture, and - fre- 

 quently very pure ; in colour it may be snowy white, pale grey, or 

 buff. The microscope reveals the fact that chalk is principally 



