98 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
flowage in a molten state. While often present without the porphyritic 
quartzes, when at all abundant and large the feldspars are almost always 
accompanied by the quartzes. These present an invariable appearance, 7. ¢., 
a glassy rough surface, which has a black color owing to the dark back- 
ground in which the erystals lie. They are rarely very minute, running from 
one-twentieth to one-fifteenth of an inch in diameter. Both of the porphy- 
ritie constituents vary greatly in amount, at times sinking out of sight, when 
the rock becomes a felsite, and again nearly equaling the matrix in com- 
bined quantity. 
In the thin section the matrix of these rocks is only rarely colorless, 
as seen in the ordinary light, being commonly more or less thoroughly 
stained red by minute particles of iron oxide. Some sections show red and 
white material blotched or interbanded in waving non-continuous bands. 
Very characteristic are the deep-brown, and deep-red to black, translucent 
to opaque, irregularly outlined, or needle-shaped ferrites. These range from 
the most minute particles to pieces which, with a low power, will run a 
quarter of the way across the field. They occur in both red and white 
portions, where the sections present a blotching or banding of these colors ; 
but are more abundant in the red. They are at times without any appa- 
rent arrangement, and again may show a crowding in the neighborhood of 
the porphyritic ingredients; while in many sections they present a pro- 
nounced linear arrangement, as is shown, for instance, in Fig. 1 of Plate XII. 
In some of the sections in which the fluidal structure is brought out in 
especial prominence by a very marked interbanding of colorless and red- 
stained portions—as for instance in those of the banded rock of the Great 
Palisades of the Minnesota coast—the ferrite-needles are abundant and 
large-sized in the latter bands, and, while showing a general tendency to 
follow the courses of the bands, they yet lie across each other at small 
angles. The appearance produced is strongly suggestive of the common 
brush-fence, a simile used by Zirkel in describing a similar arrangement of 
ferrite-needles in some of the western rhyolites.* Figs. 3 and 4 of Plate 
XII illustrate this peculiarity. 
The red-stained material which makes up the whole groundmass of 
1 Geological Exploration of the Fortieth Parallel, Vol. VI. 
