DETRITAL ROCKS. 
129 
Tabulation of microscopic observations upon sandstones, &e:—Continued. 
Place. 
| Specimen num- 
ber. 
| 
1852..| Near Copper Falls 
mine, Keweenaw 
Point, Michigan. 
Macroscopic descrip- 
tions. 
| 
AE} 
~~ 
na : 
2 & 
Fa :| a 
oO aio 5 
Bo | & 
2 {Sil 2 
<7 alal| «@ 
NE. | 12} 58 |31W. 
Medium-grained; 
dark purplish-red ; 
of open texture; 
effervesces briskly 
with hydrochloric 
acid; rather more 
coarsely grained 
than 1943; contains 
an occasional peb- 
ble of felsite or por- 
phyry. 
Microscopic descriptions. 
graphic secondary quartz, which is 
characteristic of that rock throughout 
the Lake Superior region. There are 
also present rarer, but not uncommon, 
particles worn from basic rocks. 
These are recognizable as for the 
most part derived from the matrices 
of amygdaloids, showing the peculiar 
feldspar microliths and deep-brown 
ferritic alteration characteristic of 
these matrices. There are also 
present, however, particles plainly 
derived from the more completely 
crystalline portions of the basic rocks, 
but always ina much altered condi- 
tion. There are here and there 
magnetite particles. The constituent 
particles range, for the most part, 
from 0.55™™ to 0.70™= in length. 
There is little or no original intersti- 
tial matter; but the spaces between 
the particles are often occupied by 
infiltrated calcite, with a little quartz, 
both presenting themselves in very 
fine particles. 
The particles of which this rock is com 
posed are from angular to sub-angu- 
lar. Although generally showing 
some signs of attrition, they are 
never much rounded. They often 
reach 1.00™™ or more in greatest 
length. They have been derived, for 
the most part, from the matrix and 
porphyritic feldspars of a quartzifer- 
ous porphyry; but particles referable 
to the augite-syenites are common, 
showing every phase of the character- 
istic secondary quartz of these rocks ; 
whilst smaller particles from the 
basic rocks, as in 1943, are not wholly 
wanting. Quartz particles, plainly 
derived from the quartzes of a quartz- 
iferous porphyry, occur, but are not 
abundant. There is no original inter- 
stitial matter; but the often wide 
spaces between the grains are now 
wholly occupied by a coarsely crys- 
talline calcite. Fig. 3, Plate XVL 
