PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 511 



Diabase.] 



Mey. A very dark-gray diabasic rock, of rather fine grain. 

 Mic. The section shows a somewhat altered diabase, quite similar to some of 

 the preceding (Nos. 717 and 719). Secondary quartz is common. 

 One section examined. 

 Af/e. Cabotian. u. s. o. 



No. 721. DIABASE (ivith quarto arid hornblende). 



A north and south section was taken of the steep, northward facing hill 250 feet in hight, which is crossed 

 by the portage between Gunflint and Loon lakes. The section was made a little east of the portage trail and 

 was thus near the eastern sides of sees. 23 and 26, T. 65-3 W. Rocks Nos. 721-727 come from this hill. No. 721 

 forms the top of the hill and is seventy-five feet thick. 



Ref. Annual Report, ix, page 81; Annual Report, x, pages 86, 87; Bulletin ii, page 75. 



Mi-;/. A dark-gray rock, of medium grain, and thus somewhat coarser than the 

 preceding diabases. It is composed of gray feldspar and black minerals, among which 

 are magnetite and hornblende. The feldspar has a tendency to a flattening and 

 elongation in one plane, giving a semblance of a platy texture to the rock. 



^^i<. M. E. Wadsworth's description of this section is as follows:* 



"A dark-green section, composed of partially altered augite, dialfage, feldspar, 

 rniKjtietiti', and secondary quartz, biotite, hornblende, riri/lite, and apatite. The section 

 is stained in places yellowish from ferric oxide. The pyroxene is altered along its 

 edges and even often throughout its interior tp viridite, biotite and hornblende 

 (both green and brown). The alteration extends generally along the cleavage 

 planes, which in the diallage appear to be produced from a change in the common 

 augite. The feldspar is kaolinized and contains quartz and other secondary minerals. 

 The rock, mineralogically, could well be pronounced a hornblende-biotite-granite 

 bearing accessory pyroxene, although it is evidently an altered basalt of the gabbro 

 or diabase type." 



One section examined. 



Aye. Cabotian. u. s. G. 



No. 722. DIABASE ( iritli i/i/m-h: ami hornblende}. 



Prom about half way down the perpendicular portion of the bluff. Same locality as No. 721. 

 Ref. Annual Report, ix, page 81; Annual Report, x, page 86; Bulletin ii, pages, 115, 116. 



A dark-gray rock of fine grain, composed of feldspar and black minerals. 

 Pyrite is quite common. 



Mic. M. E. Wadsworth's description of this section is as follows :f 



" The section is similar to those of Nos. 716 and 719, and it still shows in 



polarized light its original structure of divergent feldspars with the interstitial 



portions of pyroxene, etc. But the section is now largely composed of secondary 



minerals, such as chlorite, ririilite, bioflfe, <//<firfz, magnetite, hornblende, etc., with 



t iiiiii>'iin a, p. 75. 



\till/lrti,l ii, p 115. 



