PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 115 



Zirkelyte. Diabase.] 



Remarks. In the hand specimen are visible both varieties of this series of 

 rocks, viz.: the porphyritic, represented by No. 8, and the non-porphyritic, represented 

 by No. 13. They apparently grade into each other, the latter becoming very fine 

 grained, as if from contact on cooler rocks, along one side, and also assuming a darker 

 color. This rock might have the name zirkelyte, on the supposition that the matrix 

 of the feldspars was originally glassy. N. H. w. 



No. 13B. ZIRKELYTE. (Glassy basalt.) 



Duluth. A layer of No. 13. 

 Ref. Annual Report, ix, page 13. 



Mey. Earthy, amygdaloidal, thin sheeted, green, fine grained. 



Mic. The amygdules are largely occupied by quartz. The rock, in general, is 

 made up of a confused mixture of very fine crystallites which cannot be separated 

 sufficiently for determination. They seem to have prevailingly a stout and even 

 globular form, often stained by a green viriditic substance which in some places 

 renders the slide dark, even like an isotropic substance between crossed nicols. 

 Quartz is disseminated in these darkened portions of the slide, as well as in the 

 amygdaloidal spaces. 



The rock seems to have resulted from devitrification of a glassy amygdaloid, 

 of the basic sort. 



Age. Cabotian. N. H. w. 



No. 14. ZIRKELYTE. 



Duluth. A dike, breaking through No. 13, running W. 10 N. 

 Ref. Annual Report, ix, page 13. 



Meg. A compact, very fine grained, brownish rock showing minute feldspars. 



Mic. The section is composed of small lath-shaped plagioclases in a groundmass 

 of altered material which consists of magnetite, chlorite, calcite, and some feldspathic 

 material. 



Age. Manitou (?) 



Remarks. This rock is very similar to Nos. 13 and 13A, but its occurrence 

 as a dike indicates it is younger than they are. It has no porphyritic feldspars. 

 The matter between the feldspar microlites is semi-isotropic, appearing like a devit- 

 rified glass rather more than like a changed augitic mineral. N. H. w. AND u. s. G. 



No. 15. DIABASE (with olivine). 



Duluth. A dike fifteen feet wide, running N. 25 W. 

 Ref. Annual Report, ix, page 14. 



Meg. A medium grained, dense, apparently fresh, dark-colored rock glittering 

 with cleavage surfaces of the pyroxene element, and with the striated feldspar, the 

 latter, however, being less conspicuous and acicular in outline. 



