210 



PALEOZOIC TIME. 



where the latter are shales, — in other words, where the seas after- 

 ward had a muddy bottom, — there the species were almost wholly 

 different, and the new fauna was one fitted for the muddy bottom, 

 including, therefore, many Lamellibranchs with the Brachiopods, and 

 but few Crinoids. 



4. GENERAL OBSERVATIONS ON THE LOWER SILURIAN. 



Thus far in American Geology, no evidence has been detected of 

 (1) fresh-water lakes or deposits, or (2) of terrestrial or fresh-water 

 animal life. The animals were mainly Protozoan, Molluscan, and 

 Radiate, because these are the aquatic divisions of the animal king- 

 dom ; and with them were associated the aquatic Articulates, — Worms 

 and Crustaceans ; but not yet the aquatic Vertebrates, — Fishes. Ter- 

 restrial animal life may have existed, but no trace of it has yet been 

 found. The continent was already outlined, and, in its heavings and 

 progressing changes, its coming features were shadowed forth, — even 

 its mountain chains, the wide interior basin and the great lakes, — 

 although the mountains had yet but small parts above the seas, and 

 the lakes only the beginnings of their depressions. 



1. Differences in the conditions of the several continental regions 



of North America («.) Reality of the Eastern Border region in 



American geological history. — In the Primordial and Canadian periods, 

 the thickness of the limestone strata made in the Newfoundland seas 

 was far greater than that over the Continental Interior. And, during 

 the Hudson River epoch, when fragmental rocks were forming over 

 New York, a limestone formation commenced in Anticosti, which con- 

 tinued in progress through a large part of the Upper Silurian, with no 

 break at the close of the Lower Silurian. Such facts sustain the state- 

 ment, on page 145, that the Eastern Border region — including cen- 

 tral and eastern New England, and the British possessions on the 

 north to Labrador and Newfoundland — was an area of progress inde- 

 pendent of that of the great mass of the continent. 



(b.) The formations thicker in the Appalachian region than over 

 the Continental Interior. — The whole thickness of the Lower Silurian 

 in Missouri was 2,000 feet ; in Iowa, 1,200 ; in Illinois, but 700 ; in 

 Middle Tennessee, 1,000 feet, where the outcrops, however, expose 

 nothing below the top rocks of the Canadian period. On the con- 

 trary, in the Appalachian region (which includes the whole mountain 

 region from Quebec to Alabama), the thickness in Pennsylvania was 

 1 2,000 feet (Rogers) ; in the Green Mountains, not less ; in Canada, 

 north of Lake Champlain and Vermont, at least 7,000 feet ; in East 

 Tennessee, 15,000 feet, or more. 



(c.) Proportion of limestones to the sandstones and shales less in the 



