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MUSEUM OF COMPARATIVE ZOOLOGY. 



87 



Fragments of amygdaloid, sometimes rounded, at others angular, aro 

 found enclosed in the pumice-like trap, as though tliey had become 

 detached and afterwards reunited to the mass, while in a molten state. 

 Numerous short and Irregular fissures, extending to no great depth, aro 

 observed on the upper surface of the trap, in which sandstone has been 

 deposited Between the sandstone above and the trap heloiVj it is ex- 

 tremely difficult to determine where the one begins, and the other ends. 

 Fragments of amygdaloid, angular or partly rounded, are included 

 in the sandstone — more numerous near the base than at the top of the 

 deposits. Whei'c the sandstone is imposed 07i the trap, there is little 

 evidence of its having been metamorphosed ; but, on the other hand, 

 ■where the trap rests on the sandstone, the line of junction is clear and 

 well delined. The trap is less vesicular; and the upper portion of the 

 sandstone belt, for the distance of three or four feet, is converted into a 

 ribbon jasper, having a compact texture. These phenomena have been 

 observed at numerous places both on Isle Eoyale and Keweenaw Point. 

 The beds of sandstone are not shattered, nor docs the igneous rock pene- 

 trate in the form of dikes or ramifying veins. All the phenomena indicate 

 that the igneous rocks were not ]3rotruded in the form of dikes between 

 the strata, but that they flowed like lava sheets over the pre-existing 

 surface ; and that the sand was deposited in the fissures and depressiona 

 of the igneous belt, in some cases while the mass was in an incandescent 

 state." {I. c, p. 87.) The conglomerate is regarded as a volcanic tuff, 

 and the sandstone as Potsdam in age. The conglomerate of Keweenaw 

 Point and Isle Royalo " consists of rounded pebbles of trap, almost in- 

 variably 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 vol- 

 canic ash and arenaceous particles, the latter having been derived from 



the sandstone then in the progress of accumulation The trap- 



pean pebbles often attain a magnitude of eighteen inches in diameter. 

 Tlieir surfaces do not present that smooth, polished appearance which 



results from the attrition of water 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 sandstone remote from the line of volcaulo 

 foci does not exceed three hundr(^d or four hundred feet in thickness, 

 the united thickness of the conglomerate bands in the vicinity of the 



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