No. :i.] SPENCER — PALAEOZOIC GEOLOGY. 155 



from the somewhat sh.ily Hudson ojroup of the 'Cincinnati 

 Arch," or, perhaps, from the margins of Medina shales that may 

 have existed on the south-western island coast. Of course in the 

 eastern portion of the old sea much shale came from the disin- 

 tegrations of tlie other Appalachian highlands. During the 

 Medina epoch, in this region, five hundred feet of shales were 

 carried down into the eastern or north-eastern portion of the sea, 

 while only twenty feet of sediments were deposited to the south- 

 westward. 



Again, the turbid waters in tlie Clinton epoch interrupted 

 periodically the growth of impure organic calcareous beds, while 

 the western portion of the old sea was nearly free from the influx 

 of mud. 



Character of the Marine T<ife arid Origin of the Limentoties. — 

 We have observed that the greater portion of the upper beds of 

 the Niagara epoch in New York, almost all in Ontario, and the 

 greater portion in Ohio, together with a considerable portion of 

 the Clinton epoch in Canada, and all of that horizon in the 

 more south-western State, are made up of dolomitic limestones 

 of a greater or less degree of purity. Let us examine into the 

 condition of the seas and of the life that flourished at this time. 



During the earlier days of the Mediterranean sea in the Nia- 

 gara epoch, in the eastern and south-western areas, the waters 

 were of a turbid character, though freer from earthy matter in 

 its northern extension. Later, however, and during the greater 

 period of its existence, only a small amount of shaly sediment 

 was occasionally carried down, thus producing favorable condi- 

 tions for the ojrowth of marine life. 



The limestones in Canada are of a highly crystalline texture, 

 and consequently most of the traces of the organisms that con- 

 tributed to their original formation are obliterated. Out of 

 numerous specimens of rocks examined under the microscope, 

 none show any organic structure, except some parts of those beds 

 containing sjwnges or stromatopora, with here and there a place 

 where a stray fossil has escaped obliteration, in the re-srystaliza- 

 tion of the calcareous mud. In fact, as reuards both shells and 

 corals, there is seldom anvthin<r left more than their casts pre- 

 served in the stone. Even when, by chance, a portion of the 

 original bed has escaped obliteration, it has become highly crys- 

 talline. Here and there is an exception to this statement, as in 

 the case of the phosphatic shells, Lingula and Discina, in which 



