-1 





83 



BULLETIN OF THE 



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" Although the conglomerate 



trappean range on Keweenaw Point exceeds five thousand feet. As we 

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

 daloid pebbles disappear, ^nd are replaced by arenaceous and argil- 

 laceous particles." {I. c, pp, 99, 100.) 

 attains a thickness of five thousand feet, yet it by no means follows that 

 the ancient sea in which it was deposited extended to that depth. Ripple- 

 marks and clay-cracks have been observed in the upper portions of this 

 group ; the one indicates comparatively shoal water, and the other the 

 ebbing and flowing of a tide, or a change in the level of the water. The 

 inference, therefore, is, that during the deposition of the conglomerate, 

 the bed of the sea was subject to repeated elevations and depressions, 

 caused by volcanic action, and that its water obeyed the same tidal laws 

 which govern the existing oceans. These conglomerates, then, may be 

 regarded as local deposites formed along the courses of the volcanic 

 fissures by the joint agency of fire and water. When the former causes 

 operated with intensity, the materials consisted of spherical masses of 



When they acted feebly, or were quiescent, the ma- 



" We have 



lava and scoriae. 



terials became argillaceous or arenaceous." (/. o., p. 109.) 



seen that, during the deposition of the sandstone, numerous sheets of 



trap were ejected, and flowed like lava-streams ; and that the igneous 



and aqueous products were so intermingled as to present the appearance 



of having been derived from, a common origin ; and that subsequently 



the unbedded trap broke through these parallel fissures, lifting up the 



sandstones, conglomerates, and bedded traps, and causing the whole 



mass to dip at high angles.'* {I c, p. HO.) The sandstone on both 



sides of Keweenaw Point, and the trap, were regarded as making one 



geological formation. 



While the sandstone is stated to dip near the southern range of trap 

 above B(;te Griae Bay 78*" southeast {l. c, p. 112), farther up Keweenaw 

 Bay the prevailing dip was said to be about 5° to the northwest (/. c, 

 p. 116). The veins were regarded as probably filled by materials *'once 

 held in aqueous solution and precipitated by electro-chemical agency," 

 while the theories of sublimation and injection were controverted (/. c, 

 pp. 174, 175). The veins were fully described so far as then known, as 

 well as the order of deposition of the minerals and associated copper, 



Prof. Louis Agassiz, in discussing the " Geological Relations of the 



r 



* See also Bull. Gcol. Soc. France, (2,) VII. 1850-51, pp. 89-100 ; Proc. Am. 

 Assoc. Adv. Sci., 1851, V. 22-38, 136-151 ; aud Am. Jour. Sci., (2,) 1851, XII. 

 222 - 239. 







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