186 



BULLETIN OF THE 



turo, 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 ohvine, and lastly the feldspar. 



This rock is similar to the celebrated iron ore of Taberg, Sweden, as 

 described by A. Sjiiren in the Geologiska Foreningens Forhandlingar 

 (187G, III. 42~G2; sec also Neues Jahrbueh lur Mineralogie, 187G, 

 434, 435). The Taberg rock has been worked as an iron ore for over 

 three hundred years. This Swedish ore is called by Sjoren " magnetite- 



olivinite." 



The feldspar is confined to the peridotite found on one side of the 

 hill, where the peridotite passes into a compact greenish-black rock, 

 showing patches of serpentine and grains of magnetite. From this fact 

 it seems necessary to regard the feklspar as abnormal and local in the 

 rock, which in general is composed of ohvine and magnetite or their 



alteration products. 



The structure remains about the same in tlie non~feldspathic portions 

 as it is in those before mentioned as holding feldspar. But the olivine 

 is entirely changed to a greenish serpentine which shows beautiful 

 fibrous polarization. The serpentine retains the form of the ohvine 

 grains, their inclusions, and the network of fissures before mentioned. 

 In some of the sections considerable carbonate was seen, presumably 

 dolomite. In one section part of the olivine grains, especially towai'ds 

 their interior, remained unchanged, but on their edges they were altered 

 to serpentine. Another change was observed here : the formation of sec- 

 ondary crystals of irregular outline that belong probably to actinolite. 

 Some are elongated and narrow ; others are short and broad, traversed 

 by cleavage planes. They evidently belong to the monoclinic system. 



The origin of this rock could not be told from its field relations, as its 

 contact with any other rock could not be found. Since the only method 

 in which its origin can be absolutely shown cannot be used without expen- 

 sive excavation, it only remains to give the pro))abilities so flir as ascer- 

 tainable from the mass itself. Such microscopic characters and mineral 

 association have been, so far as wo know, only found in eruptive rocks when 

 the origin of such rocks has been studied with sufficient care to determine 

 it. Hence we must conclude it is most probable that this mass is erup- 

 tive also, until found to be otherwise. 



It closely resembles in structure and composition some of the mete- 



fj; re 



orites, except that its iron is oxidized and not in a native state, 

 semblance which for others of the pcridotites has long been pointed out. 



