MINERALS WHICH FORM AQUEOUS ROCKS. 

 Mineral Substances which Constitute the Aqueous Rocks. 



21 



NAME. 



Quartz 



Calcite 



Clay 



Gypsui 



Dolomite 



Uock Salt 



Iron Pyrites 



Phosphatite 



Glauconite 



COMPOSITION. 



Oxide of silicon; when crystal 

 lised is in six-sided prisms 

 terminated by six-sided py 

 ramids. 



Carbonate of lime, also 

 called calcium carbonate ; 

 when crystallised is in scale - 

 nohedrons or three-faced 

 pyramids, rhombohedrons 

 or some form in the Rhom- 

 bohedral system. 



It is not, properly speaking, 

 a mineral, but a rock; when 

 pure it is identical in com- 

 position with decomposed 

 felspar, chiefly silicate of alu- 

 mina 



Hydrated sulphate of lime 



Carbonate of magnesia com- 

 bined with carbonate of 

 lime 



Chloride of sodium 



Sulphide of iron . 



An amorphous variety of the 

 mineral apatite, which is 

 phosphate of lime, other- 

 wise called tribasic calcic 

 phosphate 



In green grains, a hydrous 

 silicate of iron, alumina, 

 magnesia, soda, and potash 



MODE OP OCCURRENCE. 



In grains forming sand and sand- 

 stones, m concretions forming flint 

 and chert in limestone, more rarely 

 in crystals filling cavities in rocks 

 and concretions, and massive in 

 quartz veins and quartz rock. 



Usually amorphous, forming lime- 

 stones, and occasionally crystalline 

 as in the skeletons of echino- 

 derms, especially encrinite lime- 

 stones of the Carboniferous age. 

 It also occurs combined with clay 

 in concretions called septaria. 



In beds often made up of thin 

 layers. Slate is a hardened and 

 altered condition of clay. Shale 

 is clay hardened by pressure and 

 infiltration of mineral substances. 



Chiefly in clays in masses called 

 alabaster, and in transparent crys- 

 tals called selenite. 



Forms the magnesian limestone in 

 the Permian rocks as between 

 Nottingham and Sunderland, &c., 

 in this country ; and other rocks. 



Occurs in beds chiefly in the upper 

 part of the Trias in this country, 

 as at Droitwich, Nantwich, Shir- 

 ley wich, and near Cavrickfergus 

 in Ireland. But isolated cubic 

 crystals are found in other forma- 

 tions like the Purbeck and coal. 



Cubic crystals common in many 

 old slates. Iron pyrites or Mar- 

 casite in all clays and many lime- 

 stones. In the chalk these masses 

 are radiated and popularly called 

 thunderbolts. 



Occurs in beds of irregular con- 

 cretionary nodules in the Red 

 Crag, Coralline Crag, Upper 

 Greensand, Gault, Upper Neoco- 

 mian, Rhsetic, and Bala beds. 



Colours Bracklesham beds, Thanet 

 t*ands, Upper Greensand, Lower 

 Greensand, &c. 



