510 THE GEOLOGY OP MINNESOTA. 



[Diabase. 



by bruintixlt //<>ni/>!ende, particularly along the borders and ends of the crystal. Some 

 of this hornblende shows the well-marked prismatic amphibole cleavage. The feld- 

 spar and augite are also both altered to viridite, while secondary quartz, replacing the 

 feldspar, is abundant. In this rock we have a diabase showing a stage approaching 

 near to the formation of a quartz dioryte by alteration." 



One section examined. 



Age. Cabotian. u. s. o. 



No. 718. DIABASE (with quartz.) 



"Fine trap-rock, south side of Gunflint river, a little east of the location of No. 305 last year. This runs 

 obliquely down to the river, and under the water, in angular basaltic blocks like trap, presenting a face (back in 

 the woods) toward the north much the same as the highest bin It's of the country." Probably from near the 

 centre of the W. % sec. 16, T. 05-2 W. 



Ref. Annual Report, x, pages 85, 86. 



Mi'f/. A very dark-gray, fine-grained, diabasic rock, holding a few small 

 porphyritic feldspars. 



Mir. The section is in general similar to that of No. 717, except that this rock 

 (No. 718) is finer grained and the augite has almost entirely disappeared. Greenish 

 and grayish alteration products, the latter largely in the feldspar, are common. 

 There is one apparently pseudamygdaloidal area in which there are numerous small 

 flakes of rli'lurife in a background of calcite, which mineral also appears to make up 

 a large part of the grayish alteration products in the feldspar. 



One section examined. 



Age. Cabotian. u. s. o. 



No. 719. DIABASE (with quartz.) 



From the eastern of the two points which enclose the little bay, on the south side of Gunflint lake, from 

 which there is a portage to Loon lake. Near the centre of the western side of the S. W. J4 sec. 2), T. (15 3 W. 

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



Mey. A very dark-gray, almost black rock, of medium grain, composed of 

 feldspar and much black material among which is uidf/itt'/ifc, augite &n& hornblende. 



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



" The section is similar to that of the preceding (No. 717) but contains secondary 

 l>i ot lie from the alteration of the augite. The secondary quartz is abundant, and. 

 together with the other secondary minerals, is traversed by numerous acl'niolitc and 

 (ijittfite needles." 



One section examined. 



Ayr. Cabotian. u. s. a. 



No. 720. DIABASK (with quartz). 



From the western of the two- points which enclose the little bay, on the south side of Gunnint lake, from 

 which there is a portage to Loon lake; S. R. 14" S. E. '4 sec. 23, T. C.5 3 W. 

 Ref. Annual Report, x, pages 86, 87. 



*Hulletin ii, p. 115. 



