PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 557 



.Diabase. Gabbro. Shale.] 



No. 815. DIABASE. 



Same place as No. 814. The rock that embraces the anorthosyte masses. 

 Ref. Annual Report, x, page 111. 



Me;/. A medium-grained, yellowish-black diabase, showing " lustre-mottling " 

 indistinctly. 



Mic. A diabase composed of plagioclase laths and augite, the latter frequently 

 in large ophitic areas. There are many greenish-yellow areas (perhaps bowlingite] 

 which are thought to represent original oli vines; also secondary chlorite and some 

 magnetite. 



One section. 



Age. Cabotian. u. s. G. 



No. 816. GABBRO. 



Beaver Bay. Compare Nos. 532 and 551. 



Ref. Annual Report, x, pages 38, 41, 48, 112; Bulletin ii, page 85.* 



Meg. There are two hand samples. One is a coarse-grained gabbro composed 

 largely of plagioclase; it is similar to No. 1. The other sample is a medium-grained, 

 almost black rock with a resinous lustre. It is composed of feldspar and olivine 

 more or less altered. These are embraced in the manner of foreign pieces in the 

 trap at Beaver bay. 



Mil'. The section is made from the second hand sample and shows a forellen- 

 stein. The rock is composed essentially of plagioclase and olivine, both minerals being 

 much fissured. The olivine is in general of earlier date than the feldspar, but in 

 places they seem to have crystallized nearly simultaneously. The olivine is altering 

 along the fissures, and in some grains the alteration is complete to a yellow serpen- 

 tine-like mass. With this alteration of the olivine, considerable magnetite has been 

 developed. The feldspar commonly shows albite twinning, and, aside from the 

 fissuring, has not been altered much. Sections cut approximately normal to 010 

 show extinction angles up to 31, and a section nearly normal to c gave an extinction 

 angle of 25. Neither of these determinations was very accurate, as the sections 

 were not cut exactly in the proper directions, but the results would indicate a 

 feldspar near labradorite. 



One section. 



Age. Cabotiau. u. s. G. 



No. 817. SHALE. 



Two Harbor bay, north shore of lake Superior. 



Ref. Annual Report, x, pages 40, 114, 115. Compare No. 525. 



Meg. A gray to pink shale, the pink color being due apparently to laumontite. 

 This rock lies below the Beaver Bay diabase where charged with masses of feldspar. 



* There is some mistake about the description, given on this page, of rock No. 816. We find no section marked 816 to which 

 this description would apply, nor could a section of that nature be made from either of the hand samples marked 816. 



