8 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
tioned first, maintained the eruptive origin of the bowlder-conglomerates of 
Keweenaw Point in the following language: 
The conglomerate of Keweenaw Point and Isle Royale consists of rounded peb- 
bles of trap, almost invariably of the variety known as amygdaloid, derived probably 
from the contemporaneous lavas, and rounded fragments of a jaspery rock which may 
have been a metamorphosed sandstone, the whole cemented by a dark-red iron sand. 
This cement may be regarded as a mixture of volcanic ash and arenaceous particles, 
the latter having been derived from the sandstone then in the progress of accumula- 
tion. It is not unusual to meet with strata composed entirely of arenaceous particles 
associated with the conglomerate beds; and where these expand to a considerable 
thickness, the associated sandstone appears in alternating belts of white and red, 
and exhibits few traces of metamorphism; but where the belts of sedimentary rock 
are thin, and come in contact with the trappean rocks, the sandstone is converted into 
ajaspery rock, traversed by divisional planes, and breaking with a conchoidal fracture. 
The trappean pebbles often attain a magnitude of eighteen inches in diameter. 
Their surfaces do not present that smooth, polished appearance which results from the 
attrition of water; in fact, a close observer can readily distinguish between those 
which have been recently detached from the rock and those which have for a time been 
exposed to the recent action of the surf. 
The conglomerate appears to have been formed too rapidly to suppose that the 
masses were detached and rounded by the action of waves and currents, and depos- 
ited with silt and sand on the floor of the ancient ocean ; for, while the contemporane- 
ous sandstone remote from the line of volcanic foci does not exceed three hundred or 
four hundred feet in thickness, the united thickness of the conglomerate bands in the 
vicinity of the trappean range on Keweenaw Point exceeds five thousand feet. As we 
recede for a few miles from the line of the volcanic fissure, these amygdaloid pebbles 
disappear, and are replaced by arenaceous and argillaceous particles. We are, there- 
fore, disposed to adopt the theory, as to the origin of such masses, first suggested by 
Von Buch: “ When basaltic islands and trachytic rocks rise on fissures, friction of the 
elevated rock against the walls of the fissures causes the elevated rock to be inclosed 
by conglomerates composed of its own matter. The granules composing the sand- 
stones of many formations have been separated rather by friction against the erupted 
volcanic rock than destroyed by the erosive force of a neighboring sea. The existence 
of these friction conglomerates, which are met with in enormous masses in both hemi- 
spheres, testifies the intensity of the force with which the erupted rocks have been 
propelled from the interior through the earth’s crust. The detritus has suddenly been 
taken up by the waters, which have then deposited it in the strata which it still covers.” 
Those pebbles having a highly vesicular structure may have been ejected through 
the fissures, in the form of scoriz, while in a plastic state, and have received their 
rounded shape from having been projected through water—on the same principle as 
melted lead, when dropped from an elevation, assumes a globular form. 
In the jaspery fragments included in the conglomerate, we often observe a struct- 
ure analogous to the woody fibre of trees. These fragments are composed of lamina, 
more or less contorted, and furrowed longitudinally, like the markings in the extinct 
plants of the genus Sigillaria. A series of striz, as fine as the engraver’s lines, run 
parallel with the larger ones. These can be traced on some of the specimens, and gen- 
