230 Notice of Messrs, Foster and Whitiiey^s Report 



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/ 



The junction between the trap and conglomerate is well dis- 

 played in the vicinity of Copper Harbor. The rocks bear nearly 

 due west, with a northerly dip of 35^. The trap on the upper 

 surface resembles pumice, the vesicles frequently empty, but 

 oftener filled with calc spar, agates, chlorite, &c. Other portions 

 are wrinkled, as though arrested while flowing. The lower por- 

 tion of the conglomerate does not exhibit a clear and well-defined 

 line of demarcation, but encloses angular masses of amygdaloid, 

 as though the materials had been thrown down while the trap 

 was in a viscid state. This appearance was particularly noticed 

 a few hundred yards above Porter's island, where the pebbles, for 

 the distance of twenty feet perpendicular, are enclosed in a scori- 



aceous mass." 



On Keweenaw Point, the conglomerate rises to a height of 

 650 feet, and expands to a thickness of 4000 feet. The cul- 

 minating points in the range are back of Horseshoe harbor and 

 Grand Marais. At the base of the Porcupine mountains, the 

 conglomerate and associated sandstone have a great thickness 

 and there are numerous intercalations with trap. Near Montreal 

 river it is 2000 feet thick and the boulders are sometimes fully 

 three feet in diameter; the rock consists mostly of porphyritic 

 trappean rocks and hornblende cemented by a calcareous paste. 



The conditions or circumstances attending the formation of 

 these beds of trap and their alternations with fragmentary de- 

 posits are thus explained by the authors. 



" We may suppose that at one time all of this district formed 

 a part of the bed of the primeval ocean. Adopting the theory 

 of a cooling globe, we may further suppose that the waters were 

 in a heated condition, and differed essentiallv in chemical com- 

 position from those of the present oceans. The earth's crust 

 was intersected by numerous, powerful fissures, and the commu- 

 nication between the exterior and interior was unobstructed. 

 Tolcanic phenomena were much more frequent, and exerted on 

 a grander scale. Each volcanic paroxysm would give rise to 

 powerful currents and agitations of the water, and their abrading 



action in detaching portions of the pre-existing rocks, and depos- 



iting them in beds and layers on the floor of the ocean, would 

 operate with greater intensity than at the present time. We can 

 trace the remains of one volcanic fissure extending from the 

 head of Keweenaw Point, in a southwesterly direction, to the 

 western limits of the district; and of another, in a parallel di- 

 rection, from the head of Neepigon bay to the western limits of 



