224 PALEOZOIC TIME LOWER SILURIAN. 



unbroken fossils, there may have been shallow waters and a muddy 

 bottom, as with existing oyster-banks; where there are fine shales 

 with few fossils, or a profusion of Graptolites, the depth was pro- 

 bably somewhat greater, — for currents carry fine material farthest 

 from the shore-line ; or there may have been sheltered bays, where 

 gentle trituration would have produced the finest silt. 



The Hudson Kiver shales crossing New York east of Lake Ontario, 

 and those of Canada west of this lake, follow so exactly the same 

 course that we conclude with reason that the depression of the 

 lake had not then been formed. 



Over the interior of the Mississippi basin, away from the sub- 

 siding regions, the Trenton condition of the seas may, for the most 

 part, have continued ; and hence the continuation there of the 

 limestone formations through the Hudson period. 



The limestones of Anticosti Island, in the Gulf of St. Lawrence, 

 afford definite proof of an eastern geological basin or area, as Logan 

 has remarked. We have already spoken of this area (including 

 St. Lawrence bay and the region southwest, with part of New 

 England) as having to a great extent an independent geological 

 history ; and we shall soon have occasion to allude to it again. It 

 will be observed that the limestones of Anticosti, if of the Hudson 

 period, were in progress while the Hudson River shales were depo- 

 siting over a considerable part of the United States. 



As in the Potsdam period, the deposits of limestones and shales 

 had their greatest thickness along the Appalachian region. The 

 region, therefore, must have continued to be one undergoing great 

 changes of level, in which the amount of subsidence -mm very 

 large. 



Climate. — No proof that a diversity of zones of climate prevailed 

 over the globe is observable in the fossils of the Trenton or Hudson 

 period, or of any part of the Lower Silurian era, as far as yet 

 studied. The following species, common in the United States, and 

 occurring at least as far south as Tennessee and Alabama, have been 

 found in the strata of northern North America, near Lake Winni- 

 peg: — Strop homena alternata, Z/eptcena sericea? , Maclurea magna, Pleuro- 

 tomaria lenticularis t, Calymene senaria, Chcetetes Iqcoperdon, Receptaculites 

 Neptuni. 



The mild temperature of the Arctic is further evident from the 

 occurrence of the following United States and European species at 

 the localities mentioned on page 207: — Chcetetes Lycoperdon, Orthoceras 

 moniliforme H., Receptaculites Neptuni De France, Ormoceras crebriseptum 

 H., Huronia vertebralis Stokes ; besides Maclurea Arctica Haughton, 

 near the Chazy species M. magna. Moreover, the formation of thick 



