HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 



565 



tains, Montana, 70599 and 73138 ; and Jaguecy, Xirinca, Eio Riberia, 

 Province of Sao Paulo, Brazil, 70023. 



8. The peridotites. 



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

 constituent. 



Mineral composition. — The essential constituent is olivine associated 

 nearly alvr'ays with chroniite or picotite and the iron ores. The com- 

 mon accessories are one or more of the ferro-magnesian silicate minerals 

 angite, hornblende, enstatite aud black micaj feldspar is also present 

 in certain varieties and more rarely apatite, garnet, sillimanite, perow- 

 skite 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 : 





I. 



ir. 



III. 



IV. 



V. 



VI. 





41.58 

 0.14 



49.28 

 0.11 



4a. 84 

 1.14 



41. :t3 



1.71 



8.76 



0.42 

 0.12 



39. 103 

 4.94 



29. 176 

 3. 951 

 4.315 



11.441 

 0.446 

 0.270 



Trace. 



42.94 



10.87 



16.32 



9.07 



3.47 



10.14 



38.01 



5.32 

 23.29 

 4.11 

 0.70 

 4.92 



45. 68 



G. 28 





34. 76 





2. 15 





9. 12 





7.49 







0.26 







Trace. 

 0.15 

 0.90 











0.22 

 4.15 





Soda 











0.34 

 1.72 











1.06 

 3. 287 



5.669 

 2.93 



6.09 



2.88 



10.60 

 2.83 



1.21 





3.269 









(I) Duuite: Macon county, North Carolina. (II) Saxonite : St. 

 Paul's Rocks, Atlantic Ocean. (Ill) Picrite: Nassau, Germany. (IV) 

 Hornblende Picrite: Ty Cross, Anglesia. (V) Picrite: Little Deer 

 Isle, Maine. (VI) Lherzolite : Monte Rossi, Piedmont. 



Structure. — The structure as displayed in the different varieties is 

 somewhat variable. In the dunite it is as a rule even crystalline gran- 

 ular, none of the olivines showing perfect crystal outlines. (Specimen 

 3G845 from North Carolina.) In the picrites the augite or hornblende 

 often occurs in the form of broad plates occupying the interstices of the 

 olivines and wholly or partially inclosing them, as in the hornblende 

 picrite of Stony Point, New York, No. 38339. The saxonites and Iher- 

 zolites often show a marked porphyritic structure produced by the 

 development of large pyroxene crystals in the fine and evenly granular 

 ground mass of olivines. (Specimen 35758 from Norway. See also Fig. 

 97, as drawn by Dr. G-. H. Williams.) The rocks belong to the class 

 designated as hypidiomorphic granular by Professor Rosenbusch, that 

 is rocks composed only in part of minerals showing crystal faces pecu- 

 liar to their species. 



