FELSITE AND QUARTZ-PORPHYRY. 95 
Tabulation of results of a microscopic study of quartzless porphyries—Contiaued. 
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| 3 Place. oe Ara ee Stecropcnuis charee: Microscopic characters. 
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| 651...| Brunschweiler’s |NW...| 22 45|4 W-.| Aphanitic; brown; | Groundmass close to that of 1799, 1781 A, 
| River, Ashland carries porphyritic etc., showing rather more tabular feld- 
County, Wiscon- | feldsparsin long red spars and little or no secon lary quartz. 
sin. crystals. There is a considerable proportion of 
isotrope matter, while minute points of 
augite, scattered in the matrix, suggest 
that the abundant minute ferrites are 
alteration-products from it, as the larger 
ones evidently are. The porphyritic 
feldspars are all oligoclases, and are much 
altered to a greenish chlorite. 
3057...) Duluth, Minn....... Nw...| 27 50 |14W.! Minutely crystalline | In the ordinary light this section shows a 
to aphanitic; dark groundmass much like that of the last- 
reddish-brown; car- described rock, but in the polarized light 
ries minute red por-| it is seen to differ in containing less iso- 
phyritic feldspars. trope matter, there being a large propor- 
tion of distinctly individualized particles, 
most of which probably belong to ortho- 
clase and quartz. The latter mineral oc- 
curs frequently in good-sized clusters of 
particles. Augite particles are not un- 
common through the section. Porphy- 
ritic feldspars are in part oligoclase and 
in part orthoclase. 
1729...| Eastend of Bead Isl- |........|--------|------ Minutely crystalline; Groundmass contains little, if any, iso- 
and, mouth of Ni- light-brownish ; car- tropic substance, ‘but is made up almost 
pigon Straits, On- ries very abundant eutirely of relatively coarse particles of 
tario, Canada. porphyritic red orthoclase and quartz, with triclinic feld- 
feldspars, up to two- spar and augite. Some, at least, of the 
tenths of an inch in quartz is secondary; magnetite is also 
length; also rarer present. Porphyritic feldspars are very |! 
and much larger large and abundant, and include both 
greenish-grey feld- oligoclase and orthoclase, the latter often 
spars, reaching five- finely twinned. Therockstands between 
tenths of an inch in the more silicious of the orthoclase-gab- 
length. Comes out bros and the quartzless porphyries. 
in thin tabular frag- 
ments. SiOs, 64.73 
per cent. 
Felsite and quartziferous porphyry—Rocks belonging under this head 
play a very important réle in the Keweenaw Series, not merely as pebbles 
and bowlders in the conglomerates, but also as extensive and widely spread 
original masses, a fact which is, as already stated, now recognized for the 
first time. 
The non-porphyritic felsites and true quartz-porphyries are here con- 
sidered together for the reason that, in the Lake Superior region, as else- 
