LYELL'S TKAVELS IN NORTH AMERICA. 391 



Lower Silurian strata of England (ii. 50.) With reference to the 

 subject of Silurian fossils, Mr. Lyell takes occasion (i. 20., ii. 51.) 

 to combat the notion that species were more cosmopolitan at this 

 early period than they are at present. 



(4) The Niagara and Clinton groups form the base of the 

 North American Upper Silurian series, and are not represented on 

 the eastern flanks of the Alleghanies, the uppermost Upper Silurian 

 bed being there in immediate contact with the Hudson river 

 series. A vast tract of country on the west of this chain is, how- 

 ever, occupied with a group consisting of these Niagara beds 

 associated with (5) the Onondaga, and (6) the Helderberg series, 

 but the sub-division is not here made out, although the 

 different members are well seen in detail on the southern shore of 

 Lake Ontario. The Hamilton group (7), the uppermost of the 

 Upper Silurian series, is everywhere seen coming out from beneath 

 the old red sandstone, which it seems to enclose as in a basin. 



The Niagara limestone and shale correspond in their fossils 

 with the Wenlock and Dudley limestone of England, and overlie 

 the Clinton group, which might almost be looked upon as lower 

 Silurian, but which it is thought better in the present state of 

 palaeozoic geology to class with the upper members of the series. 

 The Onondaga salt group (5) is a remarkable formation of red 

 and green argillaceous shale, marl, and shaly limestone, sometimes 

 of great thickness, but partially developed ; and the Helderberg 

 series (6), although consisting in the State of New York of a number 

 of distinct beds, passes so insensibly into the lower group towards 

 the west, that, as we have already observed, no well-defined line of 

 distinction can be drawn. 



The Hamilton group (7) includes some shaly and slaty beds con- 

 taining Ludlow fossils, and is widely distributed. It concludes the 

 great Silurian series of North America, concerning which it may be 

 remarked on the whole that with regard to the Alleghanies, the 

 inferior or older beds range chiefly along the eastern or south- 

 eastern flank, and are distinctly marked, while the newer groups 

 of the same series, together with the Devonian and carboniferous 

 formations, make their appearance as we proceed further west- 

 ward, (ii. 9, 10.) 



The rocks of the Devonian period — the old red sandstone 

 series of North Britain and Herefordshire — are exhibited in a 

 prominent and characteristic form, surrounding each of the great 

 coal fields of the United States, and perhaps no where more 

 strikingly than in the State of New York not far from Niagara. 

 These beds here consist of olive-coloured slate and grey sandstone, 

 containing occasionally impure coal, and in some sandstones (seen 

 near Tioga) fragments of more than one species of Holoptychius 

 have been obtained associated with a Chelonichthys of large 

 dimensions. In Ohio, at the distance of 400 or 500 miles to the 

 south-west, the author was struck by the extraordinary decrease 

 in volume of the whole group, the absence of some formations, and 

 the complete identity of those sets of strata that remained. In 



c c 4 



