071 the Lake Superior Land District 229 



At a point about half a mile to the west, numerous alternations 

 of compact trap and amygdaloid were observed, having the same 

 regularity in bedding and inclination. 



We have observed this banded structure in the igneous rocks, 

 at short intervals, from Blake^s Point nearly to Washington Har- 

 bor, a distance of forty-five miles; and throughout the entire 

 extent of the island they present a remarkable uniformity in bear- 

 ing and inclination. They were, undoubtedly, deposited at first 

 in horizontal sheets, and owe their present inclination to the same 

 upheaval which uplifted the associated sedimentary rocks. We 

 regard them as purely igneous products^ and not as the result of 

 nietamorphism," 



In the chapter on the stratified and sedimentary rocks, other 

 examples of the alternations of the trappean and detrital beds are 

 given. But before mentioning the facts, the characters of the 

 sedimentary or stratified rocks should be stated. These are com- 

 prised under three divisions; 1, Conglomerate^ not strictly a 

 sedimentary rock, but a volcanic tuff; 2, the Inferior sandstone^ 

 an equivalent, according to the authors, of the Potsdam sand- 

 stone; 3, Compact or lower magnesian limestone^ referred to the 

 age of the calciferous sandstone, chazy limestone, bird's eye and 

 Black river limestones of the New York geologists. The frag- 

 rrienis of trappean rocks in the conglomerate are often 18 inches 

 in diameter; they have a rough surface as if fresh broken, and 

 are not properly worn. 



"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 deposited with silt and sand on the 

 floor of the ancient ocean ; for, while the contemporaneous sand- 

 stone 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 feel. As we recede 

 for a few miles from the line of the volcanic fissure, these amyg- 

 daloid pebbles disappear, and are replaced by arenaceous and 

 argillaceous particles. We are, therefore, disposed to adopt the 

 Jheory, as to the origin of such masses, first suggested by von 

 Buch:* ^When basaltic islands and trachytic rocks rise on fis- 

 sures, friction of the elevated rock against the walls of the fissures 

 causes the elevated rock to be enclosed by conglomerates com- 

 posed of its own matter. The granules composing the sandstones 

 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, 

 ^hich are met with in enormous masses in both hemispheres, 



* .Geognost. Briefe, p. 76-82 



