Related Formations. 



5 



But, through Pennsylvania and all the country north and north-west 

 of itj whenever both formations appear, their line of separation is a 

 somewhat crisp one. It is evident from this that over most of the 

 watery floor there occurred almost simultaneously some great physi- 

 cal change banishing the conditions favorable for the deposition or 

 secretion of carbonate of lime and permitting the introduction and 

 tranquil precipitation of dark, carbonaceous sandy clay or fine, gritty 

 blue marl." 



These varying conditions of the formation along the line of contact 

 with the inferior and superior strata have led to considering the Utica 

 slate as an independent formation and, at the same time, as beds of 

 passage between the Trenton and Hudson River formations. Prof. 

 J. D, Dana' unites all the rocks between the Chazy limestone 

 and the Medina sandstone epochs under the term Trenton period, 

 witli the divisions of the Trenton, the TJtica, and the Cincinnati 

 epochs. 



Equivalents of the Utica Slate. 



Prof. Dana includes the Utica slate alone in the Utica epoch. 

 The writer would also include strata which are apparently equivalent 

 in time as well as stratigraphical position in the geological series. 



The extensive geographical distribution of the Utica slate, its re- 

 taining its characteristic black carbonaceous shaly character, a 

 peculiar association, and comparatively large development of or- 

 ganic forms, and an average large volume unite to indicate a pro- 

 longed continuation of conditions favorable to the depositions of the 

 sediments forming it. During the period this formation was being 

 deposited in the eastern and northern portions of the Appalachian 

 basin, - the central, western and north-western areas must have been 

 receiving a deposit of a character in accordance with the conditions 

 prevailing without the limits of the deposition of the sediments 

 forming the Utica slate. 



When investigating this subject, and after the outline of this 

 paper was sketched, the writer read a passage in the description of 

 the matiual series of Pennsylvania which indicates that Prof. H. 

 D. Rogers had considered the question in the same light many 

 years before, since which time it appears to have been overlooked. 

 He says:^ 



"The lead-bearing limestone of Wisconsin and Illinois, superior 

 1 Manual of Geology. 1874. 



^ Appalachian basin is used in this paper as including the interior continental 

 basin as was originally defined by Prof. Rogers. 

 ^ Geol. ofPenn., ii, p. 784. 1856. 



