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 

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

 because these are the aquatic divisions of the Animal kingdom ; and 

 with them were associated the aquatic Articulates, — Worms and 

 Crustaceans ; but not yet the aquatic section of Vertebrates, — Fishes. 

 Whatever terrestrial life may have existed, no trace of it has yet 

 been discovered. 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 

 tho seas, and the lakes only the beginnings of their depressions. 



1. Differences in the conditions of the several continental 

 regions of North America. — (a.) Reality of the Eastern Border 

 region in American geological history. — In the Primordial era, the 

 thickness of the limestone strata made in the Newfoundland seas was 

 far greater than that over the Continental Interior. The same was 

 true again in the Quebec period. And, finally, in the Cincinnati 

 epoch, when, after the deposition of the Trenton limestones, frag- 

 mental rocks were again forming over New York, a great limestone 

 formation commenced in Anticosti, which continued in progress to the ' 

 close of the Lower Silurian, and so on to, and through a large part of, 

 the Upper Silurian. No trace of unconformability, and no striking 

 interruption in the beds, mark the transition from the Lower to the 

 Upper Silurian. Such facts sustain the statement, on page 145, that, 

 in North American geological progress, the Eastern Border region — 

 including central and eastern New England, and the British posses- 

 sions on the north to Labrador and Newfoundland — was an area of 

 progress independent 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 



