DIABASE-PORPHYRITE. 
81 
Tabulation of the results of a microscopic study of diabase-porphyrite and ashbed-diabase. 
Specimen number. 
1963... 
Place. a Macroscopic char- | Composition as determined by in sections cut | 
3 acters. the microscope. at random in 
3 : the zone O: ti. 
2 A 
& ;|4 : Angles on| 2 . 
£ g 8 & op osite| oS 
8 io 8 sides of| 42 
I S eq p |] 
Ce lala] «a cross-hair.| E 
° ° ° 
1900...| Bohemian Range, 30 | 58 |}29W.| Aphanitic; dark | Oligoclase predominates; augite, 
Keweenaw brownish-gray ; in minute grains; magnetite ; 
Point, Michigan. of a conchoidal chlorite pseud -amygdules. 
fracture. 
Old Suffolk min- | SW. | 10 | 57 |31W.) Matrix aphanitic; | This rock presents a deeply- 5 8 
ing location, | cor, mottled dark stained groundmass, in which | 12 12 24 
Praysville, Ke- reddish-brown} are scattered very large and | 13 | 14 | 27 
weenaw Point, and black. Very abundant porphyritic oligo- 7 11 18 
Michigan. abundant red clases and orthoclases, the 5 6 ll 
porphyriticfeld-| former much the more abun- 6 7 13 
spars, which at dant of the two. In some sec- 0 6 6 
| Angle between 
maximum ex- 
tinctions of 
adjacent hemi- 
tropic bands of 
the plagioclase 
times carry na- 
tive copper and 
native silver as 
replacements. 
SiO2, 59.52 per 
cent. 
6LS 
tions the groundmass pre- 
sents much the appearance 
seen in 1799 (infra), except 
that the minute tabular feld- 
spars are more often recog- 
nizable. In these sections 
also are found the same al- 
tered porphyritic angites that 
are characteristic of 1799. In 
other sections the ground- 
mass presents a peculiar in- 
tertwisting of colorless and 
deeply brown-stained por- 
tions, often so arranged as to 
indicate flowage, and in some 
of these sections there are 
abundant, minute, elongated 
gas vesicles, now filled with 
quartz or a greenish chlorite. 
These vesicular sections are of 
specimens taken from nearer 
the upper surface of the bed. 
The proportion of unindivid- 
ualized matter is large in the 
more vesicular sections and in 
those indicating flowage; the 
deep-brown portions represent- 
ing the unindividualized mate- 
rial, the lighter portions being 
made up of completely indi- 
vidualized quartz and feld- 
spar. The rock verges close 
on the limit between the more 
acid diabase-porphyrites and 
the quartzless porphyries. 
