HUDSON RIVER GROUP. 47 



upper part. Rhynchonella capax, R. dentata, Streptelasma corniculum, Favistella stel- 

 lata, Tetradium fibratum, Oypricardites haynesi, etc., are confined to the upper part. 

 Such are a few illustrations of the changing fauna at different elevations. To 

 completely present the subject would require the enumeration of all the species. 

 Criuoids, as a rule, are limited vertically, and hence each species is sought in its 

 particular range. Species having a wide geographical distribution, and character- 

 istic of the Group are Aulopora arachnoidea, Stomatopora inflata, Orthis occident- 

 alis, 0. subquadrata, 0. retrorsa, Pterinea demissa, P. insueta, Cyclonema bilix, and 

 Glyptocrinus decadactylus. 



§ 95. With this Group the Lower Silurian closes, because at its top we have 

 the greatest break stratigraphically and palseontologically that occurs from the base 

 of the Potsdam to the top of the Lower Helderberg, and because it approaches 

 nearer the line of division established by Murchison, between his Lower and Upper 

 Silurian, than any other line, if, indeed, it is not identical with it. Wherever the 

 Hudson River has been examined on the continent, the superimposed rocks are 

 unconformable with it, no passage-beds are found, and the palseontological break is 

 almost complete. In the Western States the Niagara Group succeeds it, and rests 

 unconformably upon it. In the Eastern States it is succeeded by the Medina and 

 Clinton Groups before the Niagara is reached, but the Medina rests unconformably 

 upon it. On the Island of Anticosti it has a thickness of 950 feet, and is followed 

 by rocks apparently conformable with it, although there is an abrupt palseontolog- 

 ical break. Of 121 species known to Prof. Billings from Anticosti, 80 disappear 

 at once below the dividing line, and 41 only appear above it, where they are joined 

 by 45 species that are not found below. This palseontological break is less than it 

 is at any other known place on the contineut ; but it is so great as to show that 

 probably the strata are not strictly conformable. 



§ 96. There is an important period of time indicated by this want of conform- 

 ability and palseontological change. Vast ages must have intervened, which are not 

 represented by any known rocks on the continent. More than 400 genera have 

 been fdescribed as existing previous to this time, more than three-fourths of 

 which had become extinct. Or, in other words, less than one-fourth of the genera 

 which had come into existence prior to the close of the Lower Silurian Age continued 

 to have an existence afterward. No evidence of the existence of land-plants has 

 ever been discovered in Lower Silurian rocks. We are convinced, however, that 

 land had existed above water for ages ; that it was necessarily refreshed by sun and 

 rain, by warmth and air, and that it may have sustained some kind of land vegeta- 

 tion. If the land vegetation did not possess hard parts capable of preservation, of 

 course none will ever be found. Neither has any evidence of the existence of land 

 or fresh-water animals ofthis era ever been discovered. 



