Geology of the Lake Superior Land District. 27 
‘These strata, originally deposited in a wide ocean, everywhere 
present evidences of organic existence. It is not to be supposed 
that in tracing deposits of this kind over so wide areas, the con- 
ditions of the ocean shores would have been uniform in their ~ 
causes which gave origin to it, are of themselves evidence, a 
priori, that absolute uniformity could not prevail over so wide an 
area. We are to look, in like manner, for corresponding changes 
in organic remains. It cannot be supposed that animals, pos- 
sessing ‘certain characters and habits of life, would continue to 
live for any length of time, when the physical conditions were 
unfavorable to their existence. When we consider, also, the 
extent over which these deposits have been traced, the differ- 
ence in longitude alone would lead us to expect some differences 
in the fauna of this ancient period. We have only to compare 
this great linear development of the palwozoic strata with an 
equal extent of modern coast, to form some idea of the changes 
that might be expected to occur under similar circumstances. 
In making such a comparison, however, we shall find that the 
themselves. These changes, though gradual, and readily under- 
stood, when studied continuously, are, nevertheless, difficult of 
explanation, when seen at wide intervals, or examined at distant 
localities, 
“The general results of these examinations, as will be seen by 
the local details, have shown an increasing thickness in the Pots- 
thickness may, therefore, be exposed only at some of the points 
where the originally unequal floor presents some of its elevations, 
and thus give an erroneous idea of its actual, entire thickness. 
The calciferous sandstone, in like manner, appears to increase as 
traced from the eastern limits of the district westward, and on the 
York PPh attains a thickness equal to that which it has in New 
ork, 
“From all of the evidence, it would appear that these two groups 
Which are very intimately related to each other, have their 
most extreme tennity somewhere near the northern boundary of 
the great arch, formed by the circuits of the older formations 
