734 THE GEOLOGY OP MINNESOTA. 



[Agglomerate. Granite. 



these parts with a preponderance of the feldspathic elements all in a state of fine com- 

 minution, resolvable only in bright light by the high-power objective. Two sections. 



Age. Archean (Keewatin). 



Be mark. This agglomerate extends widely southwestwardly from this locality. 

 It has been discussed in the American Geologist (volume ix, pages 359-368), where it 

 was supposed to be of early volcanic origin, the rounded masses having the nature of 

 bombs, buried in the cotemporary pulpy product of trituration of basic ash and debris. 

 This view as to the nature of the fine material separating the bombs is sustained by 

 the general composition of the second slide above described. Examined with a low- 

 power objective its aspect is that of a devitrified basic glass, which originally existed 

 in the form of a sand, much resembling the rock No. 1419 (plate Til, figure 1), and 

 is illustrated by the photograph reproduced in plate III, figure 2. N. H. w. 



No. 1511. AGGLOMERATE. (Basic.) 



From the greenstone agglomerate at the railroad cut, Ely. Taken from the " amygdaloidal " portions of 

 some of the bombs. 



Kef. Annual Report, xviii, pages 13, 59. 



Meg. The tubes are filled with calcite and are at the circumferences of the 

 bombs, penetrating from one-fourth of an inch to two inches from the interior surface 

 radially toward the centre. Their diameter is about one-fourth of an inch and less. 



Mic. The section does not show any of the tubes, but some large areas of calcite. 

 The rock in general, seen in common light, is sprinkled rather uniformly with fine nests 

 of leucoxene, but this element also appears in minute veinlets. In the groundmass, 

 which is chloritic and rather dark, are minute microliths offeldspar( ?) twinned on 

 the albite plan, which, with a considerable decay, have apparently been extended 

 right and left by secondary feldspathic growths so as to govern extinction on either 

 side, and such microliths, as well as their enlargements, are frequently seen to present 

 a stellar and radiated structure, extinguishing with an imperfect, constant black 

 cross as they are rotated. It might perhaps be more reasonable to assume that this 

 feldspathic element is altogether original, and that the crystals were concen- 

 trated from a magma, but have since decayed. This would be in keeping with 

 their general appearance of alteration, and with the indefiniteness which char- 

 acterizes the limits of their extension into the surrounding mass. Some pyroxene is 

 seen, and in a single instance it is distinctly ophitic toward the feldspars. One 

 section. 



Aye. Archean (Keewatin). N. H. w. 



No. 1514. GRANITE. 



Hinsdale stone quarry, f. e., in the Giant's range at the crossing of the Duluth and Iron Range railway. 

 Ref. Annual Report, xviii, page 59. 



