Wadsworth.] 
196 
[Ma) T 18 , 
ritic structure owing to the presence of large triclinic feldspar 
crystals. The feldspar is locally limited and gives character to 
a portion of the rock only. The feldspar shows well marked lines 
of cleavage and fracture and is somewhat kaolinized along these 
lines. It contains a few irregular flakes of biotite together with 
grains of olivine and magnetite. The order of crystallization 
appears to have been, first the magnetite, then the olivine, and 
lastly the feldspar. The olivine, with its alteration product sur- 
pentine, is the predominant mineral, the titaniferous magnetite 
being subordinate ; that is, the ore is an olivine rock holding a vari- 
able proportion, sometimes comparatively little, of magnetite. 
This microscopic examination shows the rock to be a peridotite, 
similar to the celebrated iron ore of Taberg, Sweden. This 
Swedish ore has been worked for over three hundred years and 
was described in 1876 by A. Sjoren under the name of “magnetite 
olivinite.” 
Except on the side where the feldspar was found, microscopic 
examination showed that the olivine is generally altered to a 
serpentine, which displayed a beautiful fibrous structure in polar- 
ized light. The outline of the serpentine masses, the inclosed 
grains, and network of fissures remain the same as those observed 
in the unchanged olivine. The magnetite retains the same re- 
lation to the serpentine that it held to the olivine, and the 
general structure of the rock is unchanged. Also olivine grains 
are seen only partly altered, the edges and the borders of the fis- 
sures being metamorphosed into serpentine, while the interior 
remains nearly intact. Some other products of alteration were 
observed, such as actinolite and dolomite. 
The contact of the peridotite with the adjacent rocks could not 
be found, hence no definite statement of its origin can be made. 
Such mineralogical composition and structure have not been 
found, so far I as know, in any terrestrial rocks which have not 
proved to be eruptive when their history was studied with suffi- 
cient care. Hence it is probable that this is an eruptive rock, and 
it is necessary so to regard it until it be proved otherwise. This rock 
and other varieties of peridotite are the nearest known allies of the 
meteorites, as has long been pointed out. It is rocks of this 
character, as has been suggested by others, that give us the most 
