B. Siliceous Accumulations

The siliceous deposits of organic origin are very much less common and less extensively developed than the calcareous, because of the relatively small amount of silica which is in solution in ordinary waters, and of the comparatively few organisms which secrete shells or tests of it. Nevertheless, these beds are of sufficient importance to require mention.

Infusorial Earth is a fine white power composed of the microscopic tests, or frustules of the minute plants called diatoms. The fineness and excessive hardness of the particles make this an excellent polishing powder. Beds of this earth occur in both marine and fresh-water deposits. At Richmond, Virginia, is a celebrated deposit of this kind.

Siliceous Oozes are exceedingly rare as rocks of the land; they consist of the tests of Radiolaria, such as are now accumulating in the deeper parts of the ocean (p. 172). The only land areas in which such deposits have been found occur in certain of the West Indian Islands (Barbadoes, Cuba, and others).

Flint or Chert occurs in nodules or beds, especially in marine limestones, though it is also found among the sands and clays of certain fresh-water formations, as in Wyoming. Microscopic examination sometimes reveals the presence of sponge spicules and other siliceous organisms, but this is by no means always the case. As we have seen, the structureless cherts are believed to have been formed by chemical precipitation.

C. Ferruginous Accumulations

The iron deposits which can be referred to the activity of living creatures are of small extent and importance, but certain of the bog-iron ores are believed to be due to the agency of diatoms. Bacteria and Algae, which extract the iron from its dissolved state.

D. Carbonaceous Accumulations

The rocks of this group are formed, almost entirely, by the accumulation of vegetable matter and its progressive, though incomplete, decay under water. This decay is of such a nature that the gaseous constituents diminish, while the carbon is removed much less rapidly, consequently the proportion of the latter substance steadily rises. All the varieties of carbonaceous rocks pass into one another so gradually, that the distinction between them seems somewhat arbitrary. From fresh and unchanged vegetable matter to the hardest anthracite there is an unbroken series of transitions.

Peat is a partially carbonized mass of vegetable matter, brown or black in colour and showing its vegetable nature on the most superficial examination, though the parts which have been longest macerated are often as homogeneous and as fine grained as clay, and reveal their true nature only under the microscope.

Lignite or Brown Coal is a brown or black mass of mineralized and compressed peat, and though still plainly showing its vegetable nature, it does so less obviously than peat, being more carbonized. It is an inferior fuel, though often very valuable in regions where other fuel is scarce or entirely wanting.

Coal is a compact, dark brown or black rock, in which vegetable structure cannot be detected by the unassisted eye, though microscopic inspection seldom fails to reveal it. Coal is found in beds or strata, interstratified with shales, sandstones, and, less commonly, limestones. The different kinds of coal vary much in hardness and chemical composition, but they are all connected by intermediate gradations. Bituminous Coal has (neglecting the ash) 70 to 75 % of carbon and 25 to 30 % of volatile matters, chiefly hydrocarbons, which are driven off on destructive distillation. Under the term bituminous are included many varieties of coal, which differ much in their behaviour and in their value for different purposes. Anthracite is a hard, lustrous coal, that is nearly pure carbon (aside from the ash) and has little or no volatile matter; it burns without smoke or flame and gives an intense heat. Semibitumi-nous or Steam Coal is intermediate in character and composition between the bituminous and anthracite varieties.

Cannel Coal does not belong in the series of coals above enumerated, but forms a very distinct variety. It occurs in lenticular patches, not in beds, and is very compact, though not very hard or heavy. This coal has from 70 to 85 % of carbon and the high proportion of 6 to 7 % of hydrogen, giving off large quantities of gas when heated, and burning with a white, candle-like flame. Even with the microscope, it is difficult to detect the vegetable structure of cannel, so thoroughly has the material been macerated. Evidently, cannel is an exceptional coal and has been formed in a somewhat peculiar way. While the ordinary coals evidently represent ancient peat bogs, which by subsidence allowed the sea, or other body of water, to overflow them and were thus sealed up and buried under sedimentary deposits, cannel was formed in pools of clear water, in which vegetable matter was accumulated and very completely disintegrated. This is shown not only by the shape of the coal patches, but also by the fossil fish not infrequently found in cannel.

The following table (from Kemp) displays the composition of the typical varieties of coal, not including the ash: -