92 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
the true acid felsites in acidity (70 per cent. silica or over), and kinds 
again which are intermediate between these. There exist in the Lake 
Superior region, in fact, porphyritic rocks which range from the true basic 
kinds, with less than 50 per cent. of silica, to the very acid felsites and 
quartziferous porphyries with over 70 per cent., thus forming a continu- 
ous series. It thus becomes necessary to adopt some rather arbitrary 
divisions between the different phases. The kinds with from 50 to 60 
per cent. of silica have already been considered under the heads of 
ashbed-diabase and diabase-porphyrite, while those reaching 70 per 
cent. are taken up below with the true felsites. There yet remain the 
kinds intermediate between these, both as to silica content (60 to 70 per 
cent.) and as to their microscopic characters. These are the kinds which 
are here considered under the head of quartzless porphyry. They are, in 
fact, the semi-crystalline phases which correspond to the completely crys- 
talline augite-syenites described below. 
Macroscopically these rocks show an aphanitiec matrix of a dark red- 
dish-brown or brown color, and more or less strongly developed conchoidal 
fracture. The porphyritic feldspars vary considerably in size and abund- 
ance, but are usually minute. They show habitually a red color, and in- 
clude often striated as well as unstriated kinds. The thin sections generally 
show a reddish background, with abundant brown ferrite particles and 
needles scattered through it. In most sections more or less of this base is 
isotropic, being either cryptocrystalline or truly glassy in its nature. There 
are in some slices darker and lighter bands, plainly due to flowage. The 
darker bands are always the least crystalline. Minute tabular feldspars 
are nearly always rather abundant; far less so, however, than in the sec- 
tions of the diabase-porphyrites; but always much more so than in sec- 
tions of the acid felsites, from which they are often completely absent. 
The individualized particles are, however, for the most part irregular in 
outline, and appear to belong to both orthoclase and quartz, which latter 
mineral seems to be nearly always of a secondary origin, since it com- 
monly occurs in the characteristic ramifying forms. It is, however, far less 
abundant in the rocks here included than in the felsites and the bases of 
the quartziferous porphyries. In a few places these quartzless porphyries 
