136 . PERIDOTITE. 



Prcsque Isle, Michigan. 



65. A dark grayish-black to bkxck rock, sbowing in places the irregular sliiminer of 

 eostatite holding olivine. 



Section : grayish-green, and composed of an irregular mass of enstatite, olivine, 

 diallage, magnetite, and various secondary products like feldspar, viridite, dolomite, ser- 

 pentine, etc. The olivine with its secondary products forms in places the principal 

 portion of the section ; in other parts the enstatite and diallage are the chief minerals ; 

 while the olivine and magnetite are held in grains in the interior. The olivine crystals 

 are comparatively large, but much fissured and altered along the fissures and exterior. 

 The interior portions are clear or smoky, except wliere the olivine material has been 

 completely changed. The alteration shows in the form of cloudy bands of magnetite 

 traversing the crystal along the fissure lines, wliile a further change is shown by the for- 

 mation of greenisli and yellowish serpentine along the same lines. The change continues 

 until the olivine is entirely altered. The magnetite usually assumes an irregular grating 

 form or network extending through the serpentine, as well as being arranged in lines 

 which show the former position of the olivine fissures. 



The enstatite and diallage have a slight tinge of green, and are slightly pleochroic. 

 They are traversed by longitudinal, transverse, and ii-regular fissures, the latter being more 

 abundant in part of the diallage. They form together an irregular sponge-like mass 

 holding olivine, and tlius present a structure not unlike tliat of the Ataeania and Siberian 

 pallasites, in which they play the role of the iron. They seem to form the same identical 

 continuous mass, but with a high power the line of union can be faintly seen. It 

 probably would have never been discovered if polarized light had not directed attention 

 to it. 



The enstatite polarizes with a pale greenish tint differing but little from the natural 

 color, while the diallage shows brilliant hues of mixed yellow, red, and violet. Both are 

 more or less altered along the fissures to a greenish or a yellowisli-green serpentinous 

 product, which is dichroic, varying from a green to a yellowish-brown shade. Similar 

 dichroisin, but less marked, was observed in the serpentine of the olivine. In the highly 

 altered portions of the section are lath-sliaped crystals, branching from a centre in a fan- 

 shaped mass. These appear to be feldspars, some of which possess plagioclastic characters. 

 Associated with these occur brown biotite, a little apatite, and some augitic material. The 

 structure of tliese patches closely resembles that of some diabases. The entire section is 

 traversed in places by a pale greenish serpentine in veins holding dolomite, the latter 

 mineral occurring elsewliere in the section. Some actiuolite was observed. The mag- 

 netite is in octahedrons and irregular grains, as well as in the secondary forms before 

 mentioned. The cloudiness of the olivine seems to be due to magnetite granules. 



Besides the serpentine, a bluish-green fibrous viriditic product occurs, associated with 

 brown biotite plates in such a manner as to lead to the belief that tliis product is an 

 earlier stage in the formation of biotite, which is evidently an alteration product here. 

 The structure of the enstatite portion is shown in figure 3, Plate VII. 



73, from the same locality, is a dark grayisii-black to black rock, mottled with 

 minute specks of grayisli-white, as well as witli pyrite. Weatliers to a rusty lirown. 

 Section : of a dirty green color, and composed principally of olivine gi'ains and crystals, 

 with magnetite lield by a light green mass of enstatite, diallage, and various secondary 

 products. The alteration of the olivine is greater here, as a rule, tlian in No. 65. The 

 magnetite bands along the fissures are wider, and fewer portions are left showing the 



