CHAPTER III. 



THE PEKIDOTITES. 



Section I. — lulrodnctor//. 



The term peridotite is employed by Professor Rosenbiisch to designate 

 the pre-Tertiary terrestrial rocks that are composed essentially of olivine, 

 with or without enstatite, diallage, angite, magnetite, chromite, picotite, 

 etc.* The writer would extend it so as to include all terrestrial and extra- 

 terrestrial rocks of similar composition and structure, and all the derivatives 

 of both. The state of the iron, as in the case of the preceding species, does 

 not appear to him to be a sufficient reason for separating the rocks herein 

 described into distinct species. The descriptions will be given of the me- 

 teoric peridotites first, and of the terrestrial ones later, as a matter of 

 convenience only. The order pursued in the arrangement of the meteoric 

 peridotites will be, so far as possible, the same as that followed with the 

 terrestrial ones, — those composed of olivine first, or the dunite variety; then 

 those containing olivine and enstatite (olivine-enstatite rocks) ; then those 

 containing olivine, enstatite, and diallage, or the Iherzolite variety ; etc. 

 In each case those approaching nearest to the glassy condition will be 

 described first. Of necessity this scheme has many imperfections, owing 

 to the limited number of specimens studied microscopically. 



While the writer does not believe in the necessity or value of dividing 

 the peridotite into either species or varieties — holding that they are all 

 essentially of one type, whatever may be the especial mineral composition — 

 he recognizes the fact that, excepting himself, lithologists, universally, do so 

 divide these rocks. He then thinks it better to conform for the present to 

 that method, so fiir as seems necessary and convenient to aid the science, 

 and not to retard its progress. It seems necessary, then, in deference to the 

 prevailing sentiment, that the olivine-enstatite-bearing rocks should be sepa- 



* Mikros. Pliys. ii. 524-545. 



