MICHIGAN AND WISCONSIN. 



483 



trappean or cupriferous series was said to end, and to have abutting 

 against its edges strata of the Potsdam. The latter was also said to 

 contain fragments of the other, necessarily older, rock. These facta were 

 thought to prove that the coppcr-bcaiing rocks formed a sea-shore bhiff, 

 ftlong the base of which the sandstone was deposited with its trappean 

 fragments. In order that the reader may understand the condition of 



things at that point, it will be nciiessary for us to indicate the structure 

 of the copper-bearing rocks themselves. They consist of a series of old 

 lava flows (diabase and melaphyr), intercalated between bods of con- 

 glomerate and sandstone. The traps are known to be lava flows, by 

 the baking and induration of the immediately underlying rock; by the 

 fact that tongues and dikes extend from the overlying trap down into 

 the rock beneath ; by the scoriaceous character of the upper portion of 

 the traps, and the coarser crystallization of their lower parts ; by the 

 macroscopic and niicroscopic evidences of flowing, etc. That they in 

 each case were in situ before the immediately overlying rock was depos- 

 ited, is shown by the facts that they have not affected it in any way, and 

 that they present on their upper surface the irregularities and rounded 

 knobs which lava flows are known to have, especially when exposed to 

 water action ; by the presence of rounded fx-agments of the underlying 

 trap enclosed in the overlying conglomerate ; by the absence of frag- 

 ments of tlic overlying rock in the underlying one, and by the absence 

 of any marks of intrusion of the traps between different beds. 



Sometimes the lava flow was followed by another, without any appar- 

 ent long exposure of the former ; then again the interval between the 

 two succeeding flows was so great, that sandstones and conglomerates 

 having a thickness of from a few inches to half a mile (Marvine) were 

 deposited between them. Of course from this it followed that the 

 surface of the underlying trap suflered denudation, and that afterwards 

 the conglomerate was deposited unconformably upon dt. This was 

 the .general mode of formation throughout the scries on Keweenaw 

 Point, The general condition of things may be correctly indicated by 

 the statement, that in going from the east towards the west the cu- 

 priferous scries is found to be made up of an increasing number of lava 

 flows and a diminishing number of conglomerates, until a point is 

 reached where the volcanic activity culminated, when the flows dimin- 

 ished and the conglomerates increased, until the Western sandstone was 

 reached. It would follow from the mode of formation, that whenever a 

 sandstone or conglomerate was laid down on the trap, denudation of the 

 latter would take place and fragments of it be enclosed in the uucon- 



