PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 183 



Diabase.] 



No. 116. DIABASE (with olivine). 



A point half way between Splitrock river and Two Harbor bay, having a conspicuous basaltiform struc- 

 ture. A heavy stratum. 



Bef. Annual Report, ix, pages 30, 32; Annual Report, x, page 139; Proceedings of the American Associa- 

 tion for the Advancement of Science, vol. xxx, page 162; Bulletin ii, page 113. 



Meg. In the hand specimen this rock cannot be distinguished from No. 114. 



Mic. The following is Dr. Wadsworth's description (Bulletin ii, page 113): 



"The structure is ophiticand contains plagioclase, augite, nlirhic, iiKignrtitr, liolHf, 

 and much deep green rirn'lite. The viridite and yellowish brown biotite occur in the 

 feldspar, which is more altered than the augite. The olivines have mainly been 

 changed to dark green or dark brown masses." 



One section. 



Age. Cabotian; Beaver Bay diabase. u. s. G. 



No. 117. DIABASE. (Fine.) 



Point of Two Harbor bay; the Tioo Harbor rock, often so-called, in the field notes. 



Ref. Annual Report, ix, pages 30, 31, 40; Annual Report, x, page 141; Annual Report, xiii, pages 100 (No. 

 154), 102. 



Mm: Heavy, fine-grained, compact, brown or brownish-black, thinly-bedded 

 rock. In some places coarsely crystalline and reddish, containing small quartz geodes 

 and crystals, the last making it appear like a quartz-porphyry. These red parts 

 sometimes cross the mass in the form of veins, but not as veins. They are welded 

 on and graduate into the main mass as if due to some difference in the orientation 

 or in the manner of crystallization as well as in the composition. 



Mir. A thin section made from the brown and homogeneous portion, which 

 really makes up, here, the largest part of this rock, exhibits a very fine-grained idio- 

 morphic relation between the microliths of feldspar and the other minerals, showing 

 the rock has cooled from fusion, whatever the origin of the materials. These micro- 

 liths extinguish practically parallel to their length. 



Augite, although in small grains, the mineral that envelops partially the feld- 

 spars, can be distinguished by its behavior in ordinary light and by its cleavage and 

 color, as augite. Hematite, magnetite, chlorite and apparently a little quartz are asso- 

 ciated in making up the rest of the rock. 



A thin section from the red, quartz-bearing portion of this rock presents a 

 different aspect. It is a quartz-porphyry, the quartz crystals being mostly angular, 

 lying in the midst of finer crystals reddened by ferric oxide. Occasionally large 

 crystals of orthoclase (?) much decayed and sometimes presenting a zonal structure, 

 also appear in the finer matrix. The quartzes sometimes enclose small portions of 

 the reddened matrix. This portion of this rock seems to belong to the " red-rock " 

 series, so-called, which is an accompaniment of the anorthosyte series which is sup- 



