122 BULLETIN OF THE 



trap, all show that it is of the same geological age as the trappean 

 rocks, although younger iu poiut of time. 



The sandstones and conglomerates are evidently beach-worn, and 

 are simple shore deposits. Whence the trachytic and rhyolitic ma- 

 terial came, is a subject worthy of future investigation. They were 

 evidently lavas that had been erupted prior to the basaltic flows, and 

 a knowledge of their original position, as well as tracing the different 

 kinds to their present resting-place, would give us considerable knowl- 

 edge of the physical geography of that old sea. The conglomerates 

 occurring interbedded with the traps would of necessity be more limited 

 and variable than the lava flows themselves, for they could only be 

 deposited upon the part of the lava that lay along the shore line, and 

 even there only in those places where the conditions were favorable to 

 the accumulation and retention of detritus. The amount of conglom- 

 erate would in some measure depend upon the length of time elapsing 

 between the basaltic flows; but on accovmt of the local nature of the 

 conglomerates, their absence at any particular spot does not prove the 

 immediate following of the succeeding eruption, or tliat no conglomerate 

 exists elsewhere between the two lavas. 



While we were able to do nothing to prove the geological age of the 

 eastern sandstone, it seems that Dr. Rominger has brought forward 

 evidence conclusively establishing the correctness of Messrs. Foster and 

 Whitney's view, based on its stratigraphical relations, that it was of Pots- 

 dam age. W^e have shown sufficient evidence to prove that in the parts 

 visited by us the eastern sandstone conformably underlies the trap, and 

 that, as held by Messrs. Foster and Whitney, the eastern and western 

 sandstones, and the traps lying between them, are of the same geologi- 

 cal age. Whether the idea is correct or not, as held by these gentle- 

 men, that the eastern sandstone at the Bohemian and Porcupine Moim- 

 tains is younger in order of time of deposition than the traps, and was 

 originally continuous with the western sandstone, we cannot say, having 

 never studied the sandstone in either locality. It is, however, plain that 

 such is not the relation of the eastern sandstone in the vicinity of 

 Torch Lake to either the traps or western sandstone. It is also evident 

 that the volcanic action began gradually the same as it ended, and pro- 

 duced similar alternations of trap and sandstone on both sides of Ke- 

 weenaw Poiut. 



