322 Lozvcr Silurian of Wisconsin and Minnesota. 



Lower Blue including the bed mentioned above, the Upper Buff 

 and Upper Blue beds. But neither there nor in the Rock river 

 valley do any strong evidences appear to prove that the Upper 

 Buff and Upper Blue beds are equivalent to the Trenton shales 

 of Minnesota. Only their position suggests that they are very 

 probably equivalents. 



The Trenton shales are about eighty feet thick at Saint Paul. 

 In Goodhue county they are about the same. In Fillmore county 

 they are much thinner mainly on account of the upper strata, as 

 exposed in Saint Paul, being here represented by fifteen or twenty 

 feet of limestone. So far as known the beds of the shales were 

 co-extensive in deposition in Minnesota. 



The first ten feet of Trenton shales has been described above 

 as the Stictoporella bed. It is very fossiliferous ; Orthis sub- 

 aequata var., gibbosaBiW., Anoloteichia impolitaUlr., Pachydictya 

 foliata Ulr., and Stictoporella frondifera Ulr., occur in masses. 

 The last named is unknown except in this bed and is very widely 

 distributed ; and for that reason the name Stictoporella has been 

 proposed for this ten feet of shale. 



The next thirty feet is of uniform dark green unctuous shale 

 with numerous fossils, but many of them poor on account of the 

 nature of the matrix. But here and there, especially toward the 

 top of the bed, are reefs of bryozoa and brachiopoda, as well as 

 scattering individuals of mollusca, molluscoidea and coelenterata 

 throughout. 



These reefs occur as crystalline slabs from one to four inches 

 thick and of various extent ; from their appearance they are easily 

 taken for sedimentary strata. But they have probably been 

 formed by the infiltration of calcium carbonate into lenticular 

 beds of fossils, as shown by the irregular cementing together of 

 the fossils and more particularly by being made up almost en- 

 tirely of animal remains. Also where a large shell lies horizon- 

 tally near the under surface of a slab there will usually be a shaly 

 spot or core under the shell as if it had shed off the infiltration 

 from above. Mollusca are preserved as blue calcareous casts. 



I could scarcely characterize this bed by enumerating the 

 species which occur in it. An undescribed rhynchonella, which is 

 essentially R. increbescens Hall without the concentric lines, oc- 

 curs from the top of this bed downwards. R. ainsliei Winchell 

 has a like distribution, but is more local. Only three species of 

 orthis, O. snbaeqiiata Con., O. testudinaria Dalman (variety) and 



