138 PERIDOTITE. 



66, from the same locality, is a dark greenish-brown rock traversed by a network of 

 oil-green serpentine veins, giving it a rouglily foliated appearance. The section is very 

 similar in character to that of the preceding. 



70 is a dirty green rock coming from the upper portion of the mass at Presque Isle. 

 Section grayish, and composed principally of a granular mixture of dolomite and serpen- 

 tine witli some ferruginous and micaceous materials, etc. 



72 is from a portion of the Presque Isle peridotite which has been so filled by 

 dolomitic material as to form a vein. The rock is brownish-gray with a sliglit pinkish 

 tinge, and composed of granular dolomite holding masses and grains of the decomposed 

 peridotite. It presents the usual structure observable in veins formed by the decom- 

 position and the partial removal of the original rock, and its replacement by vein 

 material. This more properly comes later, in the portion of this work in which the vein- 

 stones are described ; but on account of its connection with the previously described peri- 

 dotite it is given here. The sections are composed of a dirty gray granular dolomite, 

 liolding patches of ferruginous material, both dark and light yellowish-brown. In places 

 tiie darker portions show the cherry-red color of hematite, and appear in jaart at least to 

 replace olivine. 



The Presque Isle peridotite was microscopically studied by Dr. A. Wichmann, who 

 gave a short description of it under the name serpentine, stating tliat it consisted only of 

 olivine, serpentine, and magnetite.* No chromite was found, on making proper tests, 

 in the Presque Isle rock, either 1iy Professor Whitney f or myself;} but Dr. Eominger 

 states § that it contains two per cent of cliromic iron in small octahedrons readily attracted 

 by the magnet, although he does not remark whether this iron was tested to prove the 

 presence of chromium or not. 



This peridotite is confidently believed to be eruptive from the following observed evi- 

 dence. On the soutlieasteru side the overlying Potsdam sand.stone dips quite irregularly 

 from twenty to thirty degrees southerly. " Its strata follow continuously the curve of the 

 underlying peridotite, and even in places form anticlinals. The surface of the peridotite 

 is an irregular knobby one, while it forms as a whole an immense knob. To this knobliy 

 structure tlie layers of the sandstone conform continuously, as layers of blankets would, 

 and they show no signs of deposition against or around the knobs, but rather a structure 

 as if the sandstone layers had themselves been indented and bent by the peridotite itself 

 The sandstone with its conglomeritic portion for some two or three feet above the perido- 

 tite has been greatly indurated and changed, showing heat action, particularly that of 

 thermal waters. It is filled with vein and clialcedonic quartz ; and indurated and red- 

 dened as such rocks are known to be when in contact with eruptives of later date than 

 themselves. These indurated portions show, on examination under the microscope, that 

 much of the quartz is a secondary water deposit formed since the deposition of tlie frag- 

 ments composing the rock. Above this indurated portion comes the ordinary unaltered 

 sandstone. No fragments of the peridotite could be found macroscopically in the field or 

 microscopically in the laboratory in the sandstone. Now the sandstone does not hold 

 such relations to the Azoic rocks of tlic district when in contact with them, and it seems 

 right to maintain that this peridotite is younger and an eruptive rock, intruded in tiie 

 form of a laccolite since the Potsdam sandstone was laid down,|| 



* Geol. Wise, 1880, iii. 618. t Am. Jour. Sci., 1S59, xxviii. IS. 



i Bull. Mus. Comp. ZdoL, ISSO, TU. 01. § Geol. Mich., 18S1, iv. 136. 



II See Poster and Whitney, Geology of Lake Superior, 1S51, ii. 17, IS, 92, 131, 123 ; Bull. Mus. Comp. 

 Zool., 1880, vii. 2, 3. 6, 9, 10, 23, 60-66. 



