PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 265 



Diabase.] 



similar to that of the plagioclases, except that the striations are confined to the 

 central portions of the grain. These manifesta kind of multiple twinning sometimes 

 possessed by augite. According to Rosenbusch these twin striationsare parallel to 100. 



Magnetite is common in large angular grains, apparently of original generation. 



Apatite is abundant in the feldspar, in fine needles. These needles are frequently 

 abundant along a side which lies adjacent to the original magma unindividualized 

 as if they projected from the magma into the feldspars. They can also be seen in 

 the unindividualized magma, independent of the feldspars, sometimes forming a 

 loose network. The augite is entirely free from them. This seems to indicate that 

 they were the earliest of the generations from the magma, and that their formation 

 ceased before the augite was generated, or that the augite had a special facility for 

 rejecting them. 



If any olivine be present it is in very small amount, and is in such obscured 

 relations to the opaque remnants of the magma that it cannot be distinguished as 

 such. Dr. Wadsworth calls attention to the similarity of this rock to the ovifak 

 basalt of Greenland. 



One section. 



Age. Cabotian. 



Remark. This rock resembles the rocks Nos. 133, 126, 114, 107, 106, etc., and is 

 presumed to occupy a similar relation to earlier eruptives, if not a cotemporary in 

 time with those. It rises back of Sickle and Double bays, and forms an important 

 constituent in the topography as well as in the origin of the Sawteeth hills, both at 

 the eastern and the western extremities of that range. (Compare No. 540; also, 

 Nos. 1813 and 1815.) 



It seems to be necessary to trace the outcrop of the basal conglomerate of the 

 Potsdam, seen at Grand Portage bay and in Puckwunge valley, south westwardly, in 

 order to determine its stratigraphic relation to the great diabase, Nos. 221, 222, 540, 

 and hence to the Beaver Bay and Duluth equivalents of the same, *. e., to ascertain 

 the possible later date of this diabase than that of mount Josephine, etc. N. H. w. 



No. 222. DIABASE. (Coarse.) 



Underlies and apparently forms a part of No. 221, the two rising in a bluff about fifteen feet, and forming 

 two or three little points within a mile east of the Brule\ The intervening bays are occupied by large rounded 

 boulders of the same (i. e. of No. 221), with little rock exposure. 



Ref. Annual Report, ix, pages 56, 57; Bulletin ii, page 77, figure 2, plate VI. 



Meg. This is coarse and similar to No. 221. It weathers into a light green, and 

 sparkles all over with what at a distance appears to be mica, but which is, in part at 

 least, due to the glittering cleavages of feldspar, and of pyroxene slightly iron-stained. 



Mic. The feldspar shows a curious alteration from one species to another. The 

 original feldspar is altered by the entrance of many kaolinic microliths, and in some 



