122 THE GEOLOGY OP MINNESOTA. 



[Tuff. 



Mir. Only two small fragments of the porphyritic feldspars are seen in the 

 section. These are untwinned and are considerably altered; one contains a grain of 

 epidote. The amygdules are filled largely with quartz, with some epidote and a little 

 di tori f<-. One amygdule contains some colcite. The rest of the rock is composed of 

 minute feldspar laths and a very fine grained confused mass of magnetite, chlorite, 

 epidote and opaque matter. It seems probable that the mass of the rock outside of 

 the feldspars was originally a glass. 



One section. 



Age. Cabotian. u. s. G. 



No. 20A. TUFF. 



Duluth. Embraced in No. 20. 

 Ref. Annual Report, ix, page 14. 



An aphanitic, brownish gray rock, looking some like a hardened shale. 

 It seems to be the same as No. 8A, which see. 

 No section. 



Age. Cabotian. u. s. G. 



No. 21. TUFF. 



Duluth. Overlies No. 20, extending fifteen feet. 

 Ref. Annual Report, ix, page 14. 



Meg. Laminated like a shale, also fine grained, of a light green color, but 

 stained in spots with iron oxide. Apparently a fragmental rock. 



Mic. This rock varies in relative prevalence of the fragmental parts, making 

 it appear varigated with light and dark. Scattered throughout are sharply angular 

 bits of quartz lying in a confused, fine matrix of granular devitrified glass. This 

 glassy element is sometimes much finer than throughout the most of the section, 

 having been originally in the form of an ash or dust. Again, on the other hand, 

 some areas in the section are remarkably coarse, but these all consist of the same 

 elements. The glassy particles are sub-rounded, clouded by hematite or by other 

 opaque impurities, or are changed to a green chloritic substance. In all cases it is 

 practically dark continually between crossed nicols. Rarely a feldspar microlite 

 appears in one of these grains, and still more rarely they seem to be of the nature 

 of lapilli. Such is a grain in which is much magnetite, with only a few translucent 

 specks. In one translucent, quartz-looking grain an interference figure indicating 

 the bisectrix ( ) was discovered in making examination in convergent light. 



One section. 



Age. Cabotian. 



Remark*. This is not a typical volcanic tuff, but a subaqueous deposit of 

 tuffaceous material. The contrast in contour between the sharply angular quartzes 

 (and plagioclases) and the rounded glassy matter is the most striking and inexplicable 

 feature of this rock. Indeed, it is of all these Duluth tuffs. N. H. w. 



