90 



BOCKS FORMED THROUGH IGNEOUS AGENCIES 



mineral aggregates, and even more on account of the character 

 of their alteration products. The peridotites are further of 

 interest in presenting the nearest homologues to meteorites of 

 any of our terrestrial rocks. The group includes the plutonic 

 peridohtes (serpentine in part), and effusive picrite porphy rites 

 and limhurgites. In detail these are as below: 



(1) THE PERIDOTITES 



Peridotite, so called because the mineral peridot (olivine) is 

 the chief constituent. 



Mineral Composition. — The essential constituent is olivine 

 associated nearly always with chromite or picotite and the iron 

 ores. The common accessories are one or more of the ferro- 

 magnesian silicate minerals augite, hornblende, enstatite, and 

 black mica ; and more rarely feldspar, apatite, garnet, sillimanite, 

 perowskite, and pyrite. 



Chemical Composition. — The chemical composition varies 

 somewhat with the character and abundance of the prevailing 

 accessory. The following table shows the composition of several 

 typical varieties. 



Chemical Composition op the Pebidotites 



Constituents. 



Silica (SiOa) .... 

 Alumina (AI2O3) . . 

 Magnesia (MgO) . . 

 Lime (CaO) .... 

 Iron sesquioxide (^6203) 

 Iron protoxide (FeO) . 

 Chrome oxide (CraOg) . 

 Manganese (MnO) . . 

 Potash (KgO) .... 

 Soda (ISTagO) .... 

 Mckel oxide (MO) . . 

 Water and ignition . . 



Specific gravity . . 



I 



II 



UI 



IV 



V 



41.58% 



43.84% 



39.103% 



42.94% 



38.01% 



0.14 



1.14 



4.94 



10.87 



5.32 



49.28 



44.33 



29.176 



16.32 



23.29 



0.11 



1.71 



3.951 



9.07 



4.11 



. • . 



, , 



4.315 



3.47 



6.70 



7.49 



8.76 



11.441 



10.14 



4.92 



« • • • 



0.42 



0.436 



• - • 



« * « 



% • • * 



0,12 



0.276 



Trace 



• . . 



* » • • 



ft • • * 



Trace 



0.15 



0.22 



• * 



• «f « * 



« « * • 



0.90 



4.15 



0.34 



0.51 



• <•■«> 



• • • • 



11 • • • 



1.72 



1.06 



5.669 



6.09 



10.60 



100.66% 



101.89% 



99.307% 



99.95% 



97.32% 



^ • • • 



3.287 



2.93 



2.88 



2.83 



VI 



45.68% 



6.28 

 34.76 



2.15 



9.12 



0.26 



XmAiX 



99.46% 

 3.269 



I. Dunite: Macon County, North Carolina. II. Saxonite: St. Paul's 

 BockS; Atlantic Ocean. III. Picrite: Nassau, Germany. IV. Hornblende 

 picrite: Ty Cross, Anglesia. Y. Picrite: Little Deer Isle, Maine. YI, 

 Llierzolite : Monte Bossi, Piedmont. 



