68 GEOLOGY. 



it is stated by Dana, make up ^^-ths of the rocks as a 

 whole. But there are some other elements which enter 

 into the composition of some rocks. Sulphur exists in 

 sulphate of lime or gypsum, which in some parts of the 

 earth occurs as a rock in beds of considerable extent. 

 This is the only compound containing sulphur, of which 

 any rocks are made. Sulphurets, as you saw in Chapter 

 IV., are abundant, but they are only ores in rocks, and 

 do not enter into their composition. Hydrogen is an- 

 other element that is present in some rocks. It is pres- 

 ent in the water, which is, as you have seen, a component 

 of hydrous minerals, and some of these, as gypsum and 

 serpentine, form rocks. The amount of water in gypsum 

 is 21 per cent. 



140. Compounds in the Rocks. The elements of which 

 I have spoken do not form rocks as elements, but as com- 

 pounds. Thus oxygen forms with silicon a compound, 

 silica or silicic acid ; and this silica constitutes rock of 

 itself (quartz rock) ; or makes a part of a mixture of rock, 

 as in the quartz of granite; or, uniting with some oxyds, 

 makes silicates, as in the feldspar and mica of granite, in 

 serpentine, in chlorite, etc. The most important of the 

 compounds which compose the rocks of the globe are 

 silica, carbonate of lime, and various silicates, viz., feld- 

 spar, mica, hornblende, and pyroxene. Besides these, the 

 only minerals that have any large share in making up 

 the earth's crust are chlorite, serpentine, talc, gypsum, 

 and coal. All these minerals have been sufficiently de- 

 scribed in previous chapters. 



141. Crystalline and Uncrystalline Rocks. It is plain 

 that marble is crystalline, although none of the individ- 

 ual crystals, as they were crowded together in their for- 

 mation, was completed. The glistening is occasioned 

 by the small portions of the faces of the crystals which 

 reflect the light. The same incomplete crystalline struc- 

 ture is seen in the three minerals that compose granite, 

 as we look at a fractured face of that rock. But sand- 



