PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 258 



Diabase.] 







mer of 1876, and the face of the rock shows perpendicularly about eighteen feet. It probably exists as a dike." 

 It cuts No. 201. 



Ref. Annual Report, ix, page 52; Proceedings American Association for the Advancement of Science, 

 vol. xxx, page 163; Bulletin ii, page 113. 



Meg. A medium-grained, dark diabase showing lustre-mottling. Cracks and 

 seams are slickensided, coated with chlorite, or with thin sheets of metallic copper. 

 One hand specimen, marked 200, is a mass of dark-green chloritic material, evidently 

 from one of the layers mentioned above. Along the centre of the specimen are 

 narrow seams of a soft red mineral, perhaps laumontite. 



Mic. The rock is composed ot plagioflase laths, auyite, which is frequently in 

 plates of large size, olirhtc, now largely altered to a substance which is black, yel- 

 lowish and brown, and is perhaps Imrlingite, magnetite, hematite, chlorite and a small 

 amount of metallic copper. The olivines in some instances have part of their outlines 

 conditioned by the surrounding feldspar, thus showing that some of the feldspar is 

 older than some of the olivine. 



Two sections. 



Age. Probably a part of the great Beaver Bay diabase sheet. u. s. G. 



No. 200A. DIABASE (ivith thomsonite ' and copper). 



Fall River mine, N. W. J4 sec. 24, T. 61-1 W.; a short distance up Pall river. Near the lake shore. 

 Ref. Annual Report, ix, page 52; Annual Report, x, page 139; Bulletin ii, page iii; American Association 

 for the Advancement of Science, vol. xxx, page 163. 



Meg. Inclusions or concretionary masses, apparently within No. 200, of a 

 different color and structure from No. 200 itself. 



Mic. Following is Wadsworth's description: "In the least altered and most 

 coarsely crystalline portions the rock is composed of pinkish, divergent, lath-shaped 

 feldspars, with interstitial dark material. Irregularly intermingled with this are 

 dark-brown to black or yellowish-brown masses and streaks of decomposed and sof- 

 tened rock with a hardness of about 3. In the coarsely crystalline portions occur 

 segregations of chalcedony, epidote, zeolites, etc. 



"The least altered portions of the sections are composed of augite, magnetite and 

 feldspar, with various secondary products. The augite is but little changed to 

 vifidite, and is of a clear, pale yellow or yellowish brown color. The feldspar, 

 however, has suffered much, being kaolinized, and contains viridite. Along the 

 fissures, and in the patches of kaolin and f err He occurs considerable native copper, as 

 a secondary product, or else as an infiltration. It was not observed in connection 

 with the augite, and but rarely near the magnetite, although one might naturally 

 expect to find it in connection with that mineral. The cojpper, indeed, appears 

 mainly in the interior of the feldspars during that condition of their alteration in 

 which they are brownish gray, from the dissemination of kaolin with subordinate 



