THE IGiStEOUS KOCKS. 519 



The principal lithological types recognized among- the younger green- 

 s' ues are diabases, porphyrites, and basalts. Tlie diabases are sometimes 

 nolivinitic, but more frequently they contain olivine in different stages 

 I alteration, and often small quantities of (juartz, usually in niicropeg- 

 n ttitic intergrowths with feldspar. The porphyrites are mainly diabase- 

 porphyrites in which plagioclase is the principal if not the only phenocrj^st. 

 The basalts dift'er from the diabases in the possession of a distinct groundmass. 



QUARTZ-DIABASES. 



The most characteristic of the younger greenstones ai-e those that have 

 been called quartz-diabases by Lane.' They are more frequentl}- found 

 west of the area discussed in this monograph than within it, though an 

 excellent representative of the type is the rock constituting the mass of 

 the knob on the shore of Lake Michigamme. In his descriptions of these 

 rocks Lane states that thev — 



are always massive, ami of a dark, black, or brownish gray color with white specks 

 of glassy, more or less lath-shaped feldspar that shows with a pocket lens twinning 

 lines on the cleavage faces. In the thin section we see that the other components 

 are magnetic iron ore and a brown augite that has more or less of a violet tinge. All 

 these components have at times their own crystalline shape, and the interstices 

 between them I have called acid interstices. They are similar to those described by 

 A. C. Lawson (American Geologist, 1891, Vol. VII, p. 153) from the Rainy Lake region. 

 Where augite conies in contact with these interstices it is coated witli a dark brown 

 hornblende like basaltic hornblende and utterly unlike uralite. When feldspar 

 adjoins them we can tell by its optical properties that from being at the center a lime- 

 soda feldspar, like labradorite, the soda more and more predominates as we go toward 

 the margin, until at the margin we often have growths springing out, to form with the 

 quartz what is known as micropegmatite. These growths sometimes fill the whole 

 remaining space. At other times there is some quartz in compact grains. Another 

 curious feature of these interstices is that they are often crossed in all directions by 

 needles of ajjatite. * * * Folia of biotite often occur in the interstices near 

 magnetite, sometimes evidently derived from it. 



These interstices occur in their most characteristic forms in the freshet rocks, and 

 their structure can not be due to weathering nor to pressure, for it occurs in rocks 

 which show no trace of pressure. It seems * * * that when the rock consoli- 

 dated there were left interstices filled with hot alkaline water or dissolved water-glass, 

 ' Report of the State Board of Geological Survey for the years 1891 and 1892, Lansing, 1893, p. 177. 



