CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 9 



the like. The nearest approach which we have at the present 

 day to chalk is probably to be found in the deposit called "ooze," 

 which forms a considerable portion of the bed of the deep At- 

 lantic, and which will be afterwards noticed at greater length. 



Limestone is a hard and compact rock, and its many 

 varieties are formed in different ways, and differ from one 

 another in more or less important points. Though the sea 

 contains carbonate of lime in solution, no marine limestones 

 appear to be formed by chemical agency alone, but in all these 

 the lime is abstracted from the sea-water by the agency of 

 marine animals. Primarily, therefore, marine limestones are 

 organically-formed rocks, consisting almost entirely of corals, 

 shells, Crinoids, and other calcareous organisms. Such lime- 

 stones may fairly be compared to the great coral-reefs of the 

 Pacific and other warm seas. It is to be remembered, how- 

 ever, that many marine limestones are secondarily mechani- 

 cally-formed rocks. In these cases, the calcareous matter of 

 the rock has been originally separated by living beings, but 

 has then been transported by the waves, or by currents, to 

 certain localities at a distance, where it has been heaped up to 

 form a bed of limestone. 



Other limestone deposits, such as the stalactites and stalag- 

 mites of caves, and the "calcareous tufa" and "travertine" of 

 some hot springs, are purely chemical in their origin, and owe 

 nothing to the operation of living beings. 



Gypsum, in chemical composition, consists of sulphuric acid 

 in combination with lime and two atoms of water ; or, in other 

 words, it is a hydrated sulphate of lime. It commonly occurs 

 as a whitish or yellowish-white rock, something like loaf-sugar 

 to look at, generally arranged in distinct beds, but sometimes 

 in irregular cakes or veins. It is palaeontologically important 

 as occasionally yielding well-preserved fossils ; but its exact 

 mode of origin has not yet been fully worked out. 



Coal is, in all those forms to which the name would ordin- 

 arily be applied, an organically-formed rock, and may be re- 

 garded as formed of compressed vegetable matter. In all its 

 varieties such as bituminous coal, anthracite, and lignite or 

 brown coal, it approximates more or less closely in chemical 

 composition to wood. It consists, namely, of from seventy 

 to eighty per cent of pure carbon, with varying quantities 

 of hydrogen and oxygen, and a small amount of earthy or 

 mineral matter which constitutes the ash. All coals occur in 

 the form of beds, intercalated with other stratified rocks ; and 

 there are innumerable gradations between pure coal, earthy 

 coal, and carbonaceous shale, till we reach ordinary shale. 



