ANNUAL REPORT FOR 1875. 75 



category, tliis fact goes far to strengthen, if not confirm, the conject- 

 ure, advanced in the preceding paragraph. 



Another fact has considerable bearing upon the same point. The 

 Superior red sandstone, wherever it borders on the trap ridges, shows 

 that it has been tilted, broken up or crushed. It therefore appears, 

 that the trap, whether erupted, or upheaved convulsively or slowly, 

 encountered this formation, in its ascent. On the contrary, as we 

 have already seen, the Potsdam exhibits undisturbed horizontal bed- 

 ding on the trap. One of two things necessarily follows: either the 

 Superior red sandstone is older than the Potsdam, or the trap rocks, 

 wherever they occur in conjunction with the Superior red sandstone, 

 are younger than where they occur in conjunction with the Potsdam. 

 The evidence is not conclusive, but it is strongly corroborative. 



Again, it is known that the Superior red sandstone has a much 

 greater thickness than the Potsdam. In the region of Montreal river, 

 its thickness has been computed at five thousand feet. By trigono- 

 metrical calculation, Mr. Sweet has found its thickness to be about 

 four thousand feet on the upper St. Croix. The Potsdam in Wiscon- 

 sin is not over nine hundred feet thick. 



Now it is evident that the Superior red sandstone must have been 

 much longer in forming than the Potsdam. As the former does not be- 

 long to a later period, a portion of it must certainly antedate the latter. 



Awaiting then, a longer and more careful study and more extensive 

 and accurate collection of facts, it may be conjectured that the lower 

 and thicker portion of the Superior red sandstone is synchronous with 

 and the equivalent of the Acadian epoch of Dawson, in the Canadian 

 survey, while the upper and thinner portion is synchronous with, and 

 the equivalent of the Potsdam sandstone of the New York geologists, 

 which is represented over a wide area of Wisconsin. The argument 

 from paleontology has no bearing on this difficult question. The red 

 sandstone of the Lake Superior region is without organic remains. 

 The Potsdam of Wisconsin, while it is rich in fossils, contains no 

 species that is identical with any species in the Potsdam of New 

 York. The epoch of the formation is determined solely on strati - 

 graphical grounds, by its relation to the underlying Archaean, and to 

 the overlying Calciferous of the Canadian group, or the Lower Mague- 

 sian limestone. 



The journey from the head of St. Croix river to Bayfield confirmed 

 previous information, that the country is covered with drift to such 

 an extent as to make satisfactory geological exploration an impossi- 

 bility. There is not an outcrop of rocks for fifty miles. Most of the 

 region is destitute of living springs and streams. Numerous depres- 



