THE TERRESTRIAL PERIDOTITES. — SAXONITE. 125 



olivine grains, but of this I have grave doubts. The characters as seen microscopically 

 do not appear to me to be those of ordinary olivine, but rather those of one or more min- 

 erals of secondary origin. That this groundmass is of secondary origin, for the most part, 

 is shown by its occurrence along the fissures in the unaltered olivines, by its relations to 

 the minerals whicli it surrounds, which are the same as those existing in other rocks 

 between the original minerals and their secondary products, and by the secondary schis- 

 tose structure. In such cases as these much depends upon the experience and esiaecial 

 kind of work that the observer has done, and unfortunately such evidence cannot be 

 placed in words so as to enable others to judge of its correctness. 



It is contrary to the laws of physics and chemistry that a mineral in altering should 

 produce itself again — there is rather a passage from an unstable compound in the condi- 

 tions in which it then is, to one more stable in the same conditions. If I am right regard- 

 ing this alteration of the olivine the resulting mineral or minerals must belong either to 

 another variety of olivine or to a distinct species. 



The actinolite, chromite, picotite, magnetite, pyrite, and serpentine, I regard in this 

 case as secondary products in the rock, and not original ones. 



As said before, in places the section shows the olivine unaltered, and having the same 

 relation between the grains that exists in other rocks when the granular structure is due 

 to crystallization from an igneous magma, and not from detrital action. M. Eenard has 

 pointed out that the actinolite is more abundant in the fine groundmass than elsewhere in 

 the sections, which is in accord with ray view of their origin. One section shows at one 

 end that it is composed chiefly of a confused mass of pale-gi-eenish monoclinic crystals, 

 showing cross fracture, and which are here referred to actinolite. 



An examination of sections from the more highly altered rock shows that on further 

 alteration the fine groundmass becomes changed from a clear to a dirty-yellowish one, but 

 slightly polarizing. The hand specimens sent me bear evidence tliat they are surface and 

 weathered specimens — to which probably much of the difficulty in their study is due; 

 for, judging from M. Eenard's descriptions, he had similar specimens to mine. In this I 

 would by no means judge of what j\I. Uenard saw, but only of the sections that I have 

 myself studied. 



It is to be hoped that should these rocks ever be visited again great pains would be 

 taken to procure specimens as deep in the solid rock as it is possible to obtain tlieui.* 



Variety. — Saxonite . 



Riissdorf, Saxon>/. 



Dathe described a peridotite from Eussdorf, Saxony, as fine-grained, and of a light- 

 green color. Olivine formed the essential portion of tlie rock-mass. This mineral was 

 sliglitly altered on its edges to a granular substance of a light-yellowish to brownish color ; 

 also, along the fissures the olivine grains are changed to a light-yellowi.sh, almost homoge- 

 neous mass. Inclosed in the olivine are black octahedral crystals of picotite or chromite. 

 The enstatite shows in colorless, finely-striated sections. Olivine in small grains and 

 small black needles was observed inclosed in the enstatite.f 



Koiihcni Nonmij. 



Helland describes somg of the peridotites from Xorthern Norway as compo.sed of fresh 

 olivine, containing picotite, together with enstatite and grains of iron ore. Serpentine 



* Science, 1883, i. 590-592. t Jveues Jalir. iliu., 1S76, pp. 233-235. 



o 



