66 L1TH0L0GICAL GEOLOGY. 



Clays which have a peculiar unctuous feel are more or less pure chemical 

 compounds, consisting of silica, alumina, and water, and related to the mineral 

 species Halloysite. Halloysite consists of silica, 41.5, alumina, 34.4, water, 24.1. 

 It results from the decomposition of feldspar. Kaolin, ox porcelain clay, is an- 

 other product of the decomposition of feldspar, consisting usually of silica, 45, 

 alumina, 40, water, 15. It is a fine white clay, and is vised for the finest porce- 

 lain. Potter's clay contains more or less of these ingredients, kaolin, halloy- 

 site, or the allied species. 



7. Materials of organic origin. 



74. The materials of organic origin — that is, those derived from 

 plants or animals — may be arranged in four groups. 



( 1. ) The calcareous, or those of which limestones have been formed : 

 namely, corals, corallines, shells, crinoids, etc. The specific gravity 

 of corals is 2.4-2.82; of shells, 2.4-2.86,— the highest from a 

 Chama (Silliman, Jr.). 



(2.) The siliceous, or those which have contributed to the silica 

 of rocks, and -may have originated flints, namely, (a) the micro- 

 scopic siliceous shields of the infusoria called diatoms, which are 

 now regarded as plants ; (b) the microscopic siliceous spicula 

 of sponges. 



(3.) The phosphatic, or those which have contributed phosphates, 

 especially the phosphate of lime, as bones, excrements, and a few shells 

 related to the Lingula. Fossil excrements are called coprolites ; or 

 those of birds when in large accumulations, guano. 



The remains of animals have also afforded traces of fluorine. 



(4.) The carbonaceous, or those which have afforded coal and resin, 

 as plants. 



Besides these, there is a fifth "kind, though of little importance 

 geologically, viz., the animal tissues themselves. Only in a few 

 cases do any of these tissues remain in fossils, except in some kinds 

 of the later geological epochs. These tissues contain traces of phos- 

 phates and fluorids. 



75. (1.) Calcareous. — The following are a few analyses: 1 and 2, corals, 

 Madrepora palmata and Astraea Orion, by B. Silliman, Jr. ; 3, a shell, by the 

 same : — 



Madrepore. Ash-tea. Shell (Chama). Oyster-shell. 



Carbonate of lime 94.81 96.47 97.00 93.9 



Phosphate and fluorids. 0.45 0.06 ^ 0.5 



Sulphate of lime V 2.60 1.4 



Earthy matters 0.30 0.74 J 



Organic tissues 4.45 2.73 0.40 3.9 



Carbonate of magnesia 0.3 



