452 GEOLOGY. 



by the Helderberg fauna, mingled with it and hence a Helderberg 

 element was present. 



As to the originating tract of the Onondaga fauna, still less is known, 

 but in its maximum extension it occupied the whole of the great interior 

 Mississippian sea, which had not been the case with either of the pre- 

 ceding faunas. A very characteristic development of this fauna is 

 found along the north side of the interior basin in the vicinity of the 

 straits of Mackinac, and the suggestion hence arises that the fauna 

 invaded the interior sea in this region from the north, along a tract 

 from which the deposits have since been removed (see map, Fig. 193) 

 Less than three hundred miles to the north, in the James Bay basin, 

 the fauna has much the same facies as that nearest it on the south. 

 While there is no positive evidence of a former connection across the 

 intervening tract, there seems to be no inherent improbability in such 

 connection. 1 Besides these geographic relations there are general 

 considerations, as will be seen later, for believing that this fauna 

 had a northern origin, and had communication with northern 

 Europe. 



Almost the only light on the originating tract of the eastern Hamil- 

 ton fauna is found in the fact that a very similar fauna lived in South 

 America (Bolivia, Brazil, and the Falkland Islands) at about the 

 same time, and that some quite peculiar forms are common to the 

 two. 2 A southern origin seems therefore the most probable, and this 

 is in reasonable harmony with the distribution of the strata contain- 

 ing it. It is hence inferred that this fauna entered from the south 

 along the Mississippi tract, which may not improbably have been 

 open at this time, as there is direct evidence of land subsidence in that 

 direction. 



Concerning the northwestern Hamilton fauna, 3 there is evidence 

 that it gradually crept down from the north, probably along the tract 

 that is now the Mackenzie valley, and that for a time it was not in 

 communication with the area occupied by the southern Hamilton, 

 but that at length it passed over the intervening barrier that seems to 

 have stretched from the old lands of Wisconsin to the similar old lands 

 of Missouri. Thence it invaded the eastern interior sea and overran 



1 Weller, Jour, of Geol., Vol. X, p. 423. 



2 Williams, Bull. U. S. Geol. Surv., No. 210, p. 79. 



3 Williams, idem. 



