136 THE GEOLOGY OP MINNESOTA. 



[Aporhyolyte. Concretion. 



No. 42. APORHYOLYTE (?) 



Duluth. Four blocks north of the depot. 



Ref. Annual Report, ix, page8 12, 17. Annual Report, x, pages 62, 109. Annual Report, xiii, page 40. 



Meg. An aphanitic rock of a reddish brown color. Scattered through it are 

 very small reddish porphyritic feldspars and irregular areas of epidote and of a darker 

 mineral, probably chlorite. 



Mic. The most striking feature of the rock is that under polarized light it 

 breaks up into irregular interlocking, often not sharply outlined, areas of quartz. 

 (This is similar to, but more pronounced than in No. 83A.) The whole rock is 

 composed of these quartz areas holding the other materials poikilitically. This 

 structure has already been described many times in altered igneous rocks; it has been 

 described by Irving* from the lake Superior rocks, and recently Miss Bascomf has 

 discussed this structure from the rocks of South mountain. In the latter article are 

 many references to other descriptions. 



The porphyritic feldspars are very much altered. They in no case show poly- 

 synthetic twinning; however, some of them are so highly altered that no trace of 

 this twinning could be expected to remain. 



A few quartz grains, not showing crystal outlines, but possibly representing 

 corroded quartz phenocrysts, occur. Frequently the quartz of the groundmass in the 

 immediate vicinity of one of these grains has the same orientation as the grain. 



Throughout the section are small particles of magnetite, chlorite and epidote, 

 and red stains and specks (hematite). 



Two sections. 



Age. Cabotian. 



Remarks. Because of the nature of the groundmass, which is similar to that 

 found in altered acid lavas, the presence of quartz which was possibly porphyritic, and 

 the apparent orthoclastic nature of the porphyritic feldspars, this rock is thought to 

 represent an altered acid lava, most probably a rhyolyte. The name aporhyolyte is 

 applied in accordance with the usage proposed by Miss Bascom4 No. 42 is similar 

 to No. 45; also No. 850. u. s. o. 



No. 42A. CONCRETION fin No. 42}. 



Duluth. 



Ref. Annual Report, ix, page 17. 



Meg. This concretion seems to grade into the mass of the rock through a 

 siliceous periphery. The concretion, which is two and a half inches in diameter, is 

 composed almost entirely of finely crystallized epidote. 



No section. 

 Age. Concretion in Cabotian rocks. u. s. G. 



* U. S. Oeol. Survey, Mon. v, pp. 99, 100, plate 13, figures 13, 14, 1885. 

 t Journal of Geology, vol. i, pp. 814-817, 1893. 

 tioc. cit.,p. 826. 



