CORRELATION OF STRATA. Clx 
In general, taking the whole area in which these Trenton formations are found in 
Minnesota, there may be said to be three grand lithologic features alternating, from 
below upward, as follows: Limestone, shale, limestone. Hitherto it has been customary 
to place in the Galena the upper limestone and in the Trenton the lower limestone, leaving 
the intervening shales in an unascertained relation. It is, however, now clear that the 
Galena alone is strictly equivalent to the Trenton limestone of New York, while the green 
shales beneath the Clitambonites bed and the limestone beneath these are to be correlated 
respectively with the Black River and Stones River or Birdseye limestones. The lower 
limestone, therefore, should no longer be spoken of as the Trenton limestone except in a 
broad sense, while the name Galena, if retained at all in this connection, should henceforth 
be used only as expressing a lithologic phase of the Trenton group and not as a distinct 
geologic horizon. 
That the Galena is simply a lithologic phase, the prevalence of which was known to 
become reduced in passing from Iowa northward into Minnesota, was recognized in some 
of the earlier reports of the survey. It fades out gradually, and shales and shaly lime- 
stone take its place. There seems to be no other horizontal lithologic change than that 
which can be attributed to varying conditions of the same oceanic expanse dependent on 
nearness or remoteness from the ancient shore line. The present surface strike of these 
formations in southern Minnesota is northerly, and in Lower Silurian time, as well as now, 
that must have been toward the ancient land area of the region. Nothing, therefore, could 
have been more natural than that the limestone phase should be replaced, at the same 
horizon, passing northward, by a lithologic phase embracing more and more of shale. 
The Black River formation is affected in the same way. Shale beds occupy the strati- 
graphic position of limestones in lowa and Wisconsin. So far, then, as the nature of the 
sediments may affect the distribution of the oceanic life of the Lower Silurian in the upper 
Mississippi valley, deep sea species would be crowded out more and more on approaching 
the latitude of the falls of St. Anthony. Such vertical oscillations as may have taken 
place in the bed of the ocean apparently were felt uniformily over the whole regicn, and 
they may be supposed to have been the prime cause of the grand vertical changes in the 
nature of the rock. These two components in the cause of faunal variation in the Lower 
Silurian rocks must both be admitted to have had their legitimate effects, but they 
operated differently. While a natural vertical succession of forms would be brought 
about by the action of one, in any certain locality, by the action of the other a lateral 
variation was caused. This lateral variation introduces such irregularity that it is plainly 
impossible to construct a stratigraphical scheme for the whole area, and consequently, it is 
difficult to assign all of the species uniformly to definite stratigraphic limits. This is true 
of those species that are easily affected by changed environment, and to a certain extent 
it is necessarily true of all the species concerned. : 
Two formations of the Hudson River period are recognized in southern Minnesota, 
namely, the Utica and Richmond groups. As a rule the two divisions are not distinguished 
