;l.l..\ II..N <.K STIIATA. CIX 



In general, taking the whole area in which th.-se Tronton formations are found in 

 Minnesota, there may be said to be three grand Uthologic features alternating, from 

 below upward, us 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 tin- 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 Uthologic 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 

 <>f the t-;irliT reporU 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 Iowa 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 uniformity over the whole region, 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 stratigraphtcal 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 th<> s|>ecies 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 



