BUILDING AND ORNAMENTAL STONES. 299 



OLIVINE. Chrysolite. Peridot.— Composition : Silicate of lime and magnesia. 

 Hardness, 6 to 7. 



Olivine is an essential constituent of basalt, dunite, limburgite, lher- 

 zolite, and picrite, and is a prominent ingredient of many lavas, diabases, 

 gabbros, and other igneous rocks, where it occurs in the form of rounded 

 blebs of a bottle-green color. It also occurs occasionally in metamor- 

 phic rocks and is a constituent of many meteorites. Olivine is subject 

 to extensive alteration, becoming changed into serpentine. Many beds 

 of serpentine result entirely from the alteration of oli vine-bear ing rocks. 



GARNET. — Composition : Variable; essentially asilicate of alumina, lime, iron, or 

 maguesia. Hardness, 6.5 to 7.5. 



This mineral is an abundant accessory in mica schist, gneiss, granite, 

 crystalline limestone, and occasionally in serpentine, volcanic tuff, and 

 lava. 



The presence of garnets in stones designed for finely finished work is 

 always detrimental, since, owing to their brittleness and hardness, they 

 break away from the stone in the process of dressing and render the 

 production of smooth surfaces a matter of difficulty. Those garnets 

 which are found in such stone as are used for building are nearly always 

 of a red color and rounded form. 



EPIDOTE. — Composition : Silica, 37.83 per cent. ; alumina, 22.63 per cent. ; iron 

 oxides, 15.98 per cent. ; lime, 23.27 per cent. ; water 2.05 per cent. Hardness, 6 

 to 7. 



This mineral is a common constituent of many granites, gneisses, and 

 schists, especially the hornblendic varieties. It is also found as a sec- 

 ondary constituent in the amygdaloidal cavities of many trap rocks, and 

 is readily recognizable from its green color. Although a common con- 

 stituent in small proportions of many rocks, those cases in which it is 

 sufficiently abundant to give them a specific character are extremely 

 rare. Certain of the New Hampshire and Massachusetts granites con- 

 tain it in such quantities as to be recognizable as greenish specks on 

 a polished surface, as does also the melaphyr quarried at Brighton, in the 

 latter State. 



CHLORITE. Viridite.— Hardness, 2 to 3. 



Under the general name chlorite are included several minerals occur- 

 ring in fibers and folia, closely resembling the micas, from which they 

 differ in their large percentage of water, and in their folia being inelas- 

 tic. The three principal varieties recognized are ripidolite, penninite, 

 and prochlorite, any one of which may occur as the essential constitu- 

 ent of a chlorite schist. Chlorite as a secondary product often results 

 from and entirely replaces the pyroxene, hornblende, or mica in rocks 

 of various kinds, and also occurs filling wholly or in part the amygda- 

 loidal cavities of trap rocks. In this form it is frequently visible only 

 with the microscope, and owing to the difficulties in the way of an exact 

 determination of its mineral species is called viridite, from the Latin 



