124 PERIDOTITE. 



iusoluble matter of the ejecta of sea-fowl, altered by exposure to the air auJ sun, and to 

 the actiou of salt and fresh water." * 



According to Kev. A. llenard, the rock is composed essentially of very small olivine 

 grains similar to those of the New Zealand dnnite. Fluid cavities were also observed. 

 Chromite (picotite) is abundant in irregular, generally lenticular grains, of a brownish- 

 yellow color. Eenard further described a pale green mineral of irregular outline, and a 

 cleavage forming an angle of 124°, which he assigned to an amphibole mineral. Ensta- 

 tite in colorless or clear greenish-yellow sections was observed, possessing an evident 

 lamellar structure. A structm-e seen in the sections by Eenard was regarded by him 

 as a fluidal structure.! 



In a later publication, M. Eenard seems to have abandoned his idea of the eruptive 

 origin of these rocks, and inclines to the view that they are formed from crystalline 

 schists, the supposed lluidal structure being really schistose structure instead. He 

 regards this peridotite as remarkably fiesh and unaltered. Color, " blackish-gray, bordering 

 green, which when deep looks perfectly black." Its component minerals, as determined 

 by M. Eenard, are olivine, chromite, actinolite, enstatite, and a pyroxenic mineral. For a 

 fuller description the reader is referred to the original papers. J 



M. Eenard thinks that the association of olivine rocks with schists proves their similar 

 origin, and tlierefore much peridotite is sedimentary ; overlooking the fact that a region of 

 eruptive rocks is one in which the sedimentary rocks are most likely to become schistose. 

 Furthermore, many eruptive rocks are schistose, through secondary changes in them after 

 eruption. Again, many eruptive rocks have associated witli them ashes and other frag- 

 mental material of eruptive character, as well as sedimentary deposits, all of which brings 

 into intimate relations metamorphosed eruptive rocks and schists. This is a case to 

 which the principles earlier given in this volume apply. It is especially difficult to see 

 how denudation could take place to the great depth in the ocean required when, as 

 M. Eenard admits, there is no evidence of depression. 



Two specimens of this rock were kindly sent me by Mr. John Murray, of the Chal- 

 lenger Expedition. One shows on the fracture a dark grayish-green color, and as ]\I. 

 Eenard remarks, closely resembles a quartzite. Weathers to a yellowish and brownish- 

 gray. The section is seen to be composed of olivine, enstatite, diallage, picotite, chromite 

 or magnetite, pyrite, actinolite, and serpentine. 



M. Eenard remarks that the minerals have their longer axes placed parallel with the 

 supposed schistose or fluidal structure. In this section the larger grains stand in every 

 direction, some of the olivine grains having their longer axes exactly at right angles to 

 one another. No structure has been observed by me that I should regard as schistose. 

 A slight schistose appearance has been produced in my judgment by the secondary altera- 

 tion of tlie rock. Fortunately, one of the specimens sent me is of the rock said by M. 

 Eenard to be entirely fresh and unaltered. He also states that the structure of this rock 

 is peculiar, and unlike that of other olivine rocks. In one section a portion of the rock is 

 onl}' slightly altered, and this portion shows the common structure of peridotites. The 

 main mass of the rock, described by M. Eenard as the groundmass, is in my opinion 

 greatly altered, and contains only the remnants of the original minerals, surrounded by 

 their alteration products. M. Eenard regards this groundmass as composed entirely of 



» Voyage of the Challenger, ii. 100-108. f Ncues Jahr. Miii., 1879, pp. 889-394. 



X Report of llio Scientific Results of the exploring A''oyage of II. M. S. Challenger, 1873-70. Narrative, 

 vol. ii. Ap[)cndix B., 29 jjp., 1 plate ; Description Lilhologique des Recifs des St. Paul, cxtrait des Annales 

 de la Societe beige Microscopic, 1882, 53 pp. 



