118 THE GEOLOGY OF MINNESOTA. 



[Diabase. Tuff. 



No. ISA. DIABASE (ivitli olivine). 

 Duluth. From the west side of the same dike as No. 15. 



An ophitic diabase like No. 15, but here the augite is wholly lost by decay, and 

 in its place is chlorite (or i>enninite) charged with magnetite. This change is not due 

 to a simple weathering, but to the activity of super-heated water during the process 

 of cooling. This magnetite is all plainly of that origin. 



One section. N. H. w. 



No. 16. DIABASE (?) (Aini/t/<l<it<>iiM.) 



Duluth. A modification of Nos. 13 and 13A, extending (next east of the brewery) 125 feet. 

 Ref. Annual Report, ix, page 14. 



Mi//. A black, apparently much decayed, aphanitic rock with epidote amygdules. 



Mic. The rock is composed of small lath-shaped plagioclases, between which is 

 largely an opaque black substance (in part may net Hi ) chlorite and some small grains 

 of epidote. This groundmass does not entirely exhibit the characters of an alteration 

 from augite, and it is not improbable that the rock was originally glassy. 



The amygdules, some at least of which appear to be pseud-amygdules, are 

 filled with epidote and quartz in small grains. 



One section. 



Age. Cabotian. u. s. G. 



No. 17. TUFF. 



Duluth. From the series east of Minnesota point, near the old breakwater, east of the Brewery creek; 

 extends twenty feet, having a dip 43 E. 



Ref. Annual Report, ix, page 14; Annual Report, x, page 140; American Association for the Advance- 

 ment of Science, vol. xxx, page 164; American Geologist, vol. xviii, pages 211-213. 



Mr;/. A firm, granular, brown, fragmental rock. 



Mic. At a glance at the thin section the fragmental character of this rock is 

 apparent. The constituent grains are of two sorts, viz., lapilli and glass. The former 

 are vesicular, their more minute cavities now being filled with a translucent mineral, 

 and their framework charged with ferruginous matter, rendering it nearly opaque. 

 The latter are similar to the former, but are sometimes translucent, or partly opaque 

 and partly translucent. They differ from the lapilli in being simple instead of com- 

 posed of a number of vesicular masses embracing several translucent areas. It 

 appears that the rock was originally essentially a glassy tuff, composed of glassy 

 vesicular lapilli and of angular or sub-rounded glass fragments. 



Between crossed nicols the whole slide is nearly dark, which is due to the opaque, 

 ferruginous products, the isotropic chloritic areas, and the possible remnants of the 

 original glass. The only light particles, as seen between crossed nicols, seem to be 

 of secondary quartz and epidote. They are very fine, and multiple, the former not 

 affording sufficient area for the action of convergent light to form an interference 

 figure. 



