72 STRUCTURAL GEOLOGY. 



the Latin for dug up), and a bed of rock containing fossils is described as 

 fossiliferous. 



The following are the chief sources of materials of organic origin : — 

 Calcareous, or the material of limestones. — The most important animal 

 sources are shells of Mollusks, Corals, Crinoids, Foraminifers or the shells 

 of ordinary Ehizopods. The sources under the vegetable kingdom are Coral- 

 lines of calcareous or semi-calcareous nature, Confervoid and other Algse, 

 some of which, as the Nullipores, have coral-like forms, while others are 

 minute and disk-shaped, as the Coccospheres or Coccoliths. 



The following are analyses : 1 and 2, Corals, Madrepora palmata and Oculina arhus- 

 cula, by S. P. Sharpies ; 3, shell of a Terehratula, by the same ; 4, shell of an oyster : — 



1. 2. 3. 4. 



Madrepora. Oculina. Terebratula. Oyster-shell. 



Calcium carbonate 97-17 95-37 98-39 93-9 



Calcimn phosphate 0-78 0-84 0-61 0-5 



Calcium sulphate — — — 04 



Magnesium carbonate — — — 0-3 



"Water and organic matters 2-81 379 1-00 3-9 



In a Millepore (?) Coral Damour found 8.51 per cent of magnesium carbonate in one 

 species, and little in others. Forchhanimer obtained 6-36 per cent of magnesium carbonate 

 from the Coral, Isis nobilis, and 2-1 per cent in the precious Coral of the Mediterranean, 

 Corallium nobile. Of the Charce, among plants, Ch.foetida affords 31-33 per cent of ash, 

 95-35 per cent of which is calcium carbonate. 



Siliceous. — The animals that secrete silica are, in the main, (1) the 

 Sponges, and (2) the Radiolarians, a radiate section of the Ehizopods ; and 

 the vegetables are chiefly the minute Diatoms and other algoid species. 



Sponges usually consist largely of line horny fibers. Those used for 

 household purposes are an exception, and are selected for this use because 

 free from such fibers, and therefore pliant and strong. The silica, when 

 present, is in spicules, bristling with horny fibers, easily detected with a good 

 pocket lens. In some species they are so abundant as to make a net-work of 

 silica, as in the pure " glass-sponge," free from all horny fiber. See page 57 

 for a figure of one of the species. 



Phosphatic. — The chief sources among animal materials are bones, teeth, 

 epidermis, and other tissues, excrements, and the shells of Linipdoi, Discinm, 

 Obolus, Pteropods, etc. ; the outer integuments or shell of Crustaceans, 

 Insects, etc. ; and those of a vegetable source are the stems, leaves, and fruit 

 of plants, especially the edible grains. The phosphoric acid is usually 

 present in combination with lime as calcium phosphate. 



Guano comes mostly from islands or coasts that have been for a long 

 time without human residents, and where birds have had undisturbed posses- 

 sion. It is made chiefly of the excrements of birds (sometimes of bats), and 

 owes its value as a fertilizer to its nitrogen and salts of phosphoric acid. But 

 water, from the rains, percolating through it carries down the soluble phos- 

 phates into the underlying material, and if this is coral rock or other loose 



