566 



REPORT OF NATIONAL MUSEUM, 1890. 



Colors— -The prevailing colors are green, greenish-gray, yellowish- 

 green, dark green to black. 



Nomenclature and classification. — Mineralogically and geologically it 

 will be observed the peridotites bear a close resemblance to the olivine 

 diabases and gabbros, from which they differ only in the absence of 

 feldspars. Indeed, Prof. Judd has shown that the gabbros and diabase 

 both in places pass by insensible gradations into peridotites through 

 a gradual diminution in the amount of their feldspathic constituents. 

 Dr. Wadsworth would extend the term peridotite to include rocks of 

 the same composition, but of meteoric as well as terrestrial origin, 

 the condition of the included iron, whether metallic or as an oxide 

 being considered by him as nonessential, since native iron is also found 

 occasionally in terrestrial rocks, as the Greenland basalts and some 

 diabases. 



In classifying the peridotites the varietal distinctions are based upon 

 the prevailing accessory mineral. We thus have : 

 Dunite, consisting essentially of olivine only. 

 ISaxonite, consisting essentially of olivine and enstatite. 

 Picrite, consisting essentially of olivine and augite. 

 Hornblende picrite, consisting essentially of olivine and hornblende. 

 Wehrlite (or eulysite), consisting essentially of olivine and diallage. 

 Lherzolite, consisting essentially of olivine, enstatite, and augite. 

 The name Dunite was first used by Hochstetter and applied to the 

 olivine rock of Mount Dun, New Zealand. (Specimen 70346.) Saxonite 

 was given by Wadsworth, rocks of this type being prevalent in Sax- 

 ony. The same rock has since 

 been named Harzburgite by 

 Kosenbusch. The name Lher- 

 zolite is from Lake Lherz in the 

 Pyrenees. 



The peridotites are, as a rule, 

 highly altered rocks, the older 

 forms showing a more or less 

 complete transformation of their 

 original constituents into a va- 

 riety of secondary minerals, the 

 olivine going over into serpen- 

 tine or talc and the augite or 

 hornblende into chlorite. The 

 more common result of the alter- 

 ation of peridotitic rocks is 

 the conversion into serpentine, 

 which takes place through the hydration of the olivine and the liber- 

 ation of free iron oxides and chalcedony. (See Fig. 97.) Recent inves- 

 tigations have shown that a large shafft of theserpentinous rocks were 

 thus originated. Hence a part of the serpentines are here exhibited. 



Fig. 97. 

 MlCRO-STIiUCTUUE OF POBPHYBITIC LHERZOLITE. 



