SECTION NEAR THE MOUTH OF SPLIT ROCK RIVER. 301 
Feet. 
I. Red felsitic porphyry.—This rock shows finely on the coast two miles above 
the mouth of Split Rock River, where it forms a very striking red 
cliff 30 to 50 feet high. Close inspection of the cliff shows run- 
ning through it a sort of banding which is brought out by narrow 
whitish strings, and also by lighter and darker shadings in the 
general red color of the mass. These bands curve up and down in 
the most irregular manner; for a while they will seem to be nearly 
horizontal, and then will suddenly change to vertical, and indeed 
to all sorts of directions within a few feet. That they represent a 
fluidal structure there can be little doubt. There is no tendency 
to split parallel to this banding. Under the microscope the thin 
sections show a matrix closely resembling that of the quartz-por- 
phyry of the pebbles of the Calumet conglomerate of Keweenaw 
Point, figured on Plate XII, Fig.2. It presents a reddish splotehy 
appearance, the minute ferrite particles which give the color being 
very irregularly distributed; and appears to be made up largely of 
what Rosenbusch calls erypto-crystalline matter. With this are par- 
ticles which may be orthoclase, and a good deal of quartz, arranged 
in the peculiar ramifying way that characterizes the secondary 
quartz of the granitic porphyries. Scattered through this ground 
mass are numerous minute black particles. The porphyritice in- 
gredients noted are comparatively small crystals of orthoclase and 
oligoclase, no quartzes having been observed. On the lake shore 
~ this porphyry has not been observed in contact with either the 
underlying or the overlying rocks, being separated from them by 
beaches. Two miles north, however, on Split Rock River, the con- 
tact with the underlying melaphyr is very nicely exposed. The 
dip here is to the east 18° to 20°, and several other observations 
along Split Rock River show the same eastern dip. At one point 
on the Split Rock River, about one and a half miles from the lake, 
the porphyry is cut by a dike, six feet wide, of a very fine dark- 
gray rock. The dike trends with the strata, nearly north and 
south, and dips at right angles to them, or 70° west. Thickness 
Of thissporphyry.]2 22 see ss cass 22 3 Soe see eee emseee 600-700 
II. Ashbed-diabase and diabase-amygdaloid.—Several layers (not more than 
four) each with an amygdaloid, of a very dense, light-gray to dark 
brownish-gray, conchoidally fracturing rock, which, in the thin see- 
tion, shows tabular oligoclase (measurements on four different 
slices failing to find any angle above 25°); augite in aggregates of 
rounded particles; and magnetite, as the chief ingredients. In 
sections of the densest kinds there are areas which have little or 
no action between the nicols, and which appear therefore to be 
glass. More or less brown ocherous matter appears in the sections, 
the amount varying directly with the amount of brown tinge pre- 
sented by the rock macroscopically. Of the two amygdaloids seen, 
