PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 527 



Porphyryte.] 



No. 751. PORPHYRYTE. (Hornblende.) 



Mallmann's peak; S. E. % sec. 30, T. 65-6 W. (See "Remark" below.) 



Ref. Annual Pveport, x, pages 92, 93; Bulletin ii, pages 124, 125, plate X, figures 1 and 2; Annual Report, xv. 

 page 367; Annual Report, xvi, page 107; Annual Report, xvii, pages 194, 196; Annual Report, xviii, page r>3; 

 Annual Report, xxi, pages 15, 55-58. 



^f|'!/. A very fine-grained, compact, purple rock, showing numerous small, 

 porphyritic crystals of hornblende sometimes associated with chlorite. 



M-ic. M. E. Wadsworth's description of this section is as follows:* 



" A greenish-gray, porphyritic rock, having a compact, greenish-gray or brown 

 base, holding porphyritic inclusions of hornblende and augite. 



" The section has a greenish-gray groundmass, holding yellowish-brown crystals 

 of hornblende, epidote and greenish pseudomorphs of chlorite. The hornblende is of 

 the usual foreign character in the andesitic rocks, having been attacked by the molten 

 magma, which has torn and eaten into the hornblende, that has its edges blackened 

 and rendered magnetic by the heating and corroding effects. Some of the horn- 

 blendes here have been broken and faulted and blackened on the broken sides, others 

 retain only a small portion of hornblende in the interior, while others are reduced 

 to a heap of opacite or magnetite grains. The chloritic pseudomorphs are composed of 

 plates and scales of chlorite with some epidote, but whether they are pseudomorphs 

 after hornblende or augite, the writer cannot determine. The epidote is in small 

 crystals and crystal aggregations of pale yellowish color, with pleochroism varying 

 from colorless to pale yellow and to a deeper yellow. The epidote is here an altera- 

 tion product, and is commonly associated with the chlorite. The groundmass is 

 altered and is now composed of chlorite scales, partially altered augite microliths 

 and granules, magnetite grains (disseminated throughout the entire groundmass), 

 feldspar, microliths, fibrous material, etc., all replacing the usual felty base of the 

 andesytes with its enclosed minerals. Here the augite, feldspar and magnetite are 

 original, and the rest secondary. 



" So far as the writer is aware, no other rocks belonging to the andesytes have 

 been heretofore recognised in Minnesota, or, indeed, about lake Superior, except one 

 described by the writer in 1880,f as none of the diabase-porphyrytes or quartzless- 

 porphyries of Irving appear to belong to the andesytes, so far as can be told from his 

 description, unless it be that from Stannard's rock. 



"Prof. N. H. Winchell has indeed observed the peculiar character of this rock, 

 stating in the annual report for 1881 : ' This rock is a peculiar porphyry. The ground- 

 mass is amorphous, and the disseminated crystals are hornblende. It is unlike 

 anything before seen.' (Page 93.) 



" The rock itself is an altered and old andesyte of the variety known as porphy- 

 ryte or hornblende-porphyryte amongst lithologists. This andesyte, in its original 



'HnHftin ii, pp. 124, 125. 



1<jeolojry of the Copper ami Iron districts of lake Superior, 1880. 



