PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 143 



Diabase.] 



feldspar, likewise charged with impurities. The rock has the appearance of a basalt, 

 but it is not impossible that it is a condition of the " black rock " mentioned under 

 No. 44. It contains, however, no observable quartz, or very little, and the feldspars 

 are distinctly idiomorphic in the midst of the other grains. 



Age. Cabotian. 



Remark. This rock falls into the category of which Nos. 6C and 7 are other 

 examples. If, as the field notes record, this falls into the unobserved interval 

 between Nos. 6C and 7, it shows a thickness of at least 200 feet of such rock. The 

 note already made, under " remarks," in connection with No. 7, is even more 

 applicable to this rock. It is necessary further to call attention to the great thickness 

 of this dense black rock. In that respect it resembles the outcrops on Piedmont 

 avenue (Nos. 1966 and 1967), and, as an eruptive amongst elastics or as a surface 

 lava, it ought to manifest considerable more variation in texture. While, therefore, 

 No. 48 is classed with Nos. 6C and 7, with the basalts, it should be noted that there 

 are certain structural reasons, and some anomalies unexplained, as yet, which render 

 that reference questionable, and which seem to point to an alliance with rock like 

 No. 44, or Nos. 1966 and 1967, /. e., to the action of the basic eruptives on the black 

 slates of the Animikie, and hence to a greater age than the Cabotian. 



lie murk. Another section, made thin, shows that this rock is a micro-crystal- 

 line basalt, in which there is still a considerable amount of isotropic glass. Among 

 the minerals may be distinguished feldspar, augite, olivine, much magnetite, while 

 an isotropic substance, probably resulting from glass, renders the whole slide very 

 nearly dark between crossed nicols. N. H. w. 



No. 49. DIABASE (witholivun). 



Duluth. Behind the M. E. church, between Second and Third streets, and Third and Fourth avenues 

 west. 



Ref. Annual Report, ix, p. 18; Annual Report, x, p. 139; Proceedings American Association for the 

 Advancement of Science, vol. xxx, page 162; Bulletin ii, p. 108. 



Meg. A rather coarse diabasic rock, of black color; some of the feldspars are 

 grayish and a very few inclined toward a pink color. 



Mic. The description of Dr. Wadsworth (Bulletin ii, page 108) is as follows: 

 "The section is granitic in texture and composed of lath-shaped, somewhat kaolinized 

 feldspar, magnetite, brownish augite, greenish pseudomorphs of serpentine after 

 olivine bearing much magnetite, apatite, viriclite, pyrite and quartz. Of these the 

 only original minerals are the feldspar, augite, olivine and part of the magnetite." 



One section. 



Aye. Cabotian; allied to the Beaver Bay diabase, u. s. G. 



