Tabulation of the results of a microse 
DIABASE-PORPHYRITE—AMYGDALOIDS. 
87 
opie study of diabase-porphyrite and ashbed-diabase 
—Continued. 
Angle between 
maximum ex- 
tinctions of | 
adjacent hemi- | 
: tropic bands of 
8 : the plagioclase | 
E Place. 8 Macroscopic char-| Composition as determined by | ft peer out 
= s ie acters. the microscope. the zone O: it. | 
a a = SS 
q aI ;|4 5 Angles on| © . 
| = 2\3 So opposite | 6 
3 S |/s|E| 8 sides of | 42 
a oe lala! & cross-hair.| © 
° ° ° 
@) South side of Aphanitic; nearly | An excessively dense rock, in| 10 il 21 
Michipicoten black; highly which are recognizable, with 
Island. conchoidal frac- a high power in the polarized 
ture; no por- light, very numerous minute 
phyritic ingre- augile grains embedded in a 
dients. non-polarizing matrix, along 
with much rarer plagioclase | 
and magnetite. The rock is 
remarkable for its relatively 
large content of augite. 
(i a |eaoccs oecese-eab es Completely apha- | Has a base which in ordinary 
nitic; jet-black ; light looks much like that of 
greasy- vitreous No. 4 of this series; but there 
luster; glass- is more non-polarizing mate- 
like fracture. rial, large areas remainirg 
Si Oz, 57.92 per completely dark between the 
cent. erossed nicols, and the pla- 
gioclases are still more mi- 
nute. Occasional minute bril- 
liant points belonging to au- 
gite are seen, as also some 
| magnetite particles. 
1 Macfarlane’s Michipicoten Collection, No. 18, ‘‘Basaltic melaphyr.” 
2Macfarlane’s Michipicoten Collection, No. 19, ‘‘ Pitchstone.” 
Amygdaloids—The flows of the finer-grained rocks are all commonly 
provided with upper vesicular portions, by the subsequent filling of whose 
vesicles, and the various degrees of alteration of whose matrices have been 
produced the manifold types of amygdaloid known in the Lake Superior 
region. The coarse rocks—olivinitic and orthoclastic gabbros—are not 
furnished with amygdaloids save when tending to a distinctly finer grain 
than usual. 
Externally, the matrix of the amygdaloid is commonly quite different 
! Pumpelly has spoken of the olivinitic fine-grained kinds, his melaphyrs, as less commonly pro- 
vided with amygdaloids than are the olivine-free diabases of the ordinary type, but in my observations 
this is only true when the melaphyrs have a distinct tendency to become coarse-grained, as in “The 
Greenstone” of Keweenaw Point. When they are fine-grained they appear to have amygdaloids quite 
as frequently as the olivine-free kinds. 
