é 
THE BEAVER BAY GROUP. 313 
not products of infiltration like the so-called dendrites, since they run 
through and through the mass of the rock. They appear to me to furnish 
one more point of resemblance between the ancient felsites and quartz- 
porphyries and the modern rhyolites, and are another proof of the erup- 
tive origin of these rocks. 
Yet another proof of an eruptive origin is furnished by the rock of the 
Fic. 22.—Flowage structure in felsite, Minnesota coast. 
lower layers of this cliff. There are here large surfaces hundreds of feet 
square in which a fluidal structure may be seen on a large scale. Light- 
colored, pale-pinkish felsite and dark-brownish felsite are twisted together 
in various curling and snake-like forms; the mass as a whole dipping, as 
usual, 15° to the southeast. The contrast between the colors is very strong. 
The lighter material is the most abundant, and includes the darker. Fig. 22 
represents an area of 20 by 28 inches. Some of the dark-colored bands 
are a foot or two in length and an inch to three inches in width, from which 
