100 BULLETIN OF THE 



disturbance, within a few miles of highly-tilted copper rocks, which 

 gave every evidence of having been elevated before the deposition 

 of the sandstones. So far as my observation has extended, this rule is 

 general ; that is, no Lake Superior sandstone, which is unmistakably 

 lower Silurian, has ever been found in any position other than nearly 

 horizontal"; — Will Mr. Brooks visit Pi-esque Isle] — "and no rock which 

 was unmistakably of the Copper series has been seen which was not 

 considerably tilted. The fact that certain sandstones belonging to the 

 copper series are very similar, if not litliologically identical with some 

 of the lower Silurian sandstones, has helped to complicate this question. 

 An interesting locality for study in this connection is the west fork of 

 the Ontonagon Rivei-, just south of the Forrest Copper Mine. I am not 

 sure but that it affords an exception to the rule above stated, as at that 

 point sandstones, apparently Silurian, dip south at an angle of 45°." 



In Prof Pumpelly's Report of the Survey of the Copper District,* 

 we find but little written by him of geological interest excepting that 

 which had been published elsewhere, and referred to in the precediug 

 pages.f The sandstone beds on the eastern side of Keweenaw Point are 

 said to slope gently to the southeast (/. c, p. 1). The chief portion 

 of the geological woi'k, and about all of any value, seems to have 

 been done by Mr. A. R. Marvine, who, judging from his work, appears 

 to have possessed the power of observing well and accurately in the 

 field. Only certain portions of his work can be pointed out here : 

 " The conglomerate beds of Keweenaw Point have been generally con- 

 sidered as mere local deposits, rapidly fading out in either direction. 

 The table would seem to show, on the contrary, that for conglomer- 

 ates they are unusually persistent, and that while a bed may thin 

 out and lose its character as a conglomerate, it may still exist even 



as a mere seam We gather from those facts that when the beds 



composing the trappean range were being originally formed, the agen- 

 cies, whatever they were, which formed what are now the melaphyrs, 

 ceased to act not only over limited but over extended areas, in one in- 

 stance at least over fifty (50) miles, and for periods of time long 

 enough to allow of the accumulation of beds of conglomerate from a few 

 to over 75 feet — in one instance over half a mile — in thickness." 

 (/. c, pp. 60, 61.) 



Mr. Marvine points out the fact that much of the amygdaloidal charac- 

 ter of these rocks is owing to chemical action upon approximately homo- 



* Geol. of Mich., Part II., 1873. 



+ Am. Jour. Sci., 1872, (3,) II. 188-198, 243-258, 347-355; III. 423-432. 



