116 Wisco72sin Academy of Sciences, Arts, and Letters. 



half forgotten theory of their Triassic age. All of these geolo- 

 gists have however considered these two groups, i. e., the cop- 

 per-bearing, and the horizontal sandstones as of the same class. 

 It is only within the last year that Messrs. Pumpelly and Brooks 

 have recorded observations, going to show an entire difference in 

 age between the two groups, and proving that the former is 

 much the older of the two.' 



^In Michigan the highly tilted beds of the Copper Bearing 

 series, which dip northward, and which form the back bone of 

 Keweenaw Point, are flanked on the south and east by hori- 

 zontal Silurian sandstones, which abut directly against their up- 

 turned edojes. These sandstones continue westward nearly to 

 the Montreal river, having the tilted beds of trap always on 

 the north. They come to an end where the belt of Copper 

 Bearing rocks on their north unites with one on the south. 

 This more southerly belt is composed like the northern one 

 of a series of traps with interstratified sandstones and conglom- 

 erates, all inclined at a very high angle. The horizontal undis- 

 turbed beds oj the Silurian sandstones, occupies then a trough be- 

 tween two lines of highly tilted beds of the Copper Bearing series. 



In Ashland county, on Silver creek, occur horizontal sand- 

 stone, and shaly sandstone, loithin a few hundred feet of Copper 

 Bearing Trap, and within two miles of vertical sandstones of the 

 same group. 



In Douglas county the horizontal sandstones are traceable 

 to within a short distance of the trap) — and sometimes to actual 

 contact, the traps here dipping, wherever dip is observable always 

 to the southward, and having no tilted sandstones and conglomer- 

 ates associated ivith them. 



The interstratification of the trap with sandstones, and 

 their mutual conforrnability — as observable everywhere on the 

 south shore of Lake Superior — proves that the trappean beds, 

 if ever thrown out in a molten condition, must have been 



lAm. Jor. Sci. June, 18T3. T. B. Brooks—" Iron Bearing Rocks.'' Michigan geologi- 

 cal survey, 1873. 



2 Brooks and Pnmpelly. 



