OF LAKE SUPERIOR. 189 
with a much smaller proportion of protoxide. Both taken together, range from 
five to twenty per cent. In all this, these rocks resemble the red sandstones of the 
older States, and differ essentially from the more common varieties of sandstone, 
appertaining, on the Mississippi and its tributaries, to F. 1, the equivalent of the 
Potsdam Sandstone of New York. This Lower Silurian rock, taking its general 
mass, is of a white or buff colour, and is a siliceous (quartzose), not an argillaceous or 
ferruginous sandstone. With the exception of subordinate beds, of red colour, 
usually in the immediate vicinity of igneous ranges, it commonly contains ninety- 
two per cent. and upwards of silica; while of alumina and oxide of iron, taken 
together, it has seldom more than three per cent. 
In determining, however, the synchronism of two or more formations, it is, as 
every geologist knows, wholly unsafe to rely upon lithological appearance and 
chemical composition, except as corroborating indications. Equivalent formations 
in distant countries, or even at comparatively short distances, vary greatly in this 
respect. Indeed, beds of strictly cotemporaneous origin (as indicated by the identity 
of their fossil remains), may, at one locality, be true limestone, at another pure 
sandstone. 
The test of superposition, on the contrary, is satisfactory and conclusive, provided 
it be unequivocally obtained. How far Dr. Norwood, who was specially charged with 
this investigation, succeeded in obtaining it, will be seen from his Report. I myself 
have had opportunity personally to witness what, even if it be not admitted to be 
unequivocal proof of the true position of these sandstones with reference to another 
well-defined formation, must certainly be taken as very strong presumptive evidence, 
not only in favour of the opinion that the sandstones of Lake Superior are of Lower 
Silurian date, but that they underlie the paleozoic base of the Mississippi Valley. 
The phenomena to which I refer, came under my notice chiefly on the upper 
waters of the St. Croix; and are especially worthy of attention in this connexion. 
It will be remembered, that in Chapter I., when speaking of the paleontology of 
F. 1, I noticed the occurrence, on the St. Croix, just below its Falls, of Lingula and 
Orbicula beds (Tab. 1, B, & F. 1, 4); and stated, that these strata, constituting 
the palaeozoic base of the Mississippi Valley, were found in horizontal layers, 
in juxtaposition with the trap range. Here we have a point of departure, deter- 
mined by unquestionable paleontological evidence; all admitting these beds to be 
of Lower Silurian date. 
Now, as the general dip, throughout this entire region of country, is southeasterly, 
it follows that, in ascending the St. Croix, above the Falls, so long as the formation 
remains continuous, we gradually reach lower members of F.1. Accordingly, we 
find, from the Falls up as high as the mouth of Snake, the white and buff quartzose 
sandstone beds of F. 1, a, preserving all their characteristic peculiarities, in place, 
at brief intervals on both sides of the river. From this point, still ascending the 
river, occurs a marked change, challenging the attention of the geologist. Instead 
of the light or yellow quartzose sandstone, which he has left behind him, he is met, 
commencing on the St. Croix about five miles above the mouth of Snake, by red 
sandstone, argillaceous, ferruginous, similar in tint and composition to the red sand- 
stones of Lake Superior, and associated, like them, with coarse red conglomerates, and 
