THE PYROXENITE-AUGITITE GROUP 



95 



lack of olivine. Following Dr. William's nomenclature, we have 

 the varieties diallagite^ hronzitite, and hyperstlienite, according 

 as the mineral diallage, bronzite, or hypersthene forms the essen- 

 tial constituent. Wehsterite is the name given to the bronzite- 

 diopside variety, occurring near Webster, North Carolina, and 

 hornhlendite to the hornblende-augite variety. The pyroxenites 

 rank, in geological importance, next to the peridotites. Through 

 processes of hydration and other chemical changes, they pass into 

 amphibolic and steatitic masses to which the name soapstone or 

 potstone is applied. These last are dark gray or greenish rocks, 

 soft enough to be readily cut with a knife and with a pronounced 

 soapy or greasy feeling; hence the name soapstone. The name 

 potstone was given on account of their having been utilized for 

 making rude pots, for which their softness and fireproof proper- 

 ties render them well qualified. Although it is commonly stated 

 in the text-books that soapstone is a compact form of steatite or 

 talc, few are even approximately pure forms of this mineral, but 

 all contain varying proportions of chlorite, mica, and tremolite, 

 together with perhaps unaltered residuals of pyroxene, granules 

 of iron ore, iron pyrites, quartz, and, in seams and veins, calcite 

 and magnesian carbonates. The variation in chemical composi- 

 tion is shown in the following analyses, I being that of a com- 

 pact, homogeneous-appearing, quite massive variety from Al- 

 berene, in Albemarle County, Virginia, and II one from Frances- 

 town, New Hampshire. 



Chemical Composition of the Soapstones 



Constituents 



Silica (Si02) 



Alumina (AlgOa) 



Ferric and ferrous iron (FegOs) and (FeO) 



Lime (CaO) 



Magnesia (MgO) 



Potash (K2O) 



SodaCNajO) 



Ignition 



(2) AUGITITE 



The effusive form, augitite, differs from the pyroxenite proper 

 mainly on structural grounds. In common with many lavas it 



