468 GEOLOGY. 



the Oriskany species lived before and where certain of the Hamilton 

 species lived afterwards. This implies that the Onondaga fauna was 

 not distributed in that direction. In the St. Lawrence embayment, 

 which had played so critical a part in the evolution of the American 

 Paleozoic life, the Onondaga fauna appears to have no distinct repre- 

 sentation. Such traces as are present appear to be only those which 

 might arise from the counter-migration of Onondaga tribes into this 

 breeding-ground of migrations. There was quite certainly connec- 

 tion with northern Europe, because a notable number of peculiar 

 types are common to the two regions. 



The whole assemblage of life displayed in this remarkable fauna 

 seems to be explicable by postulating a generating tract in the Hudson 

 Bay embayment, or in some more northerly or northeasterly region, 

 possibly on the Eurasian continent. Into this it may be assumed 

 that a portion of the Silurian fauna retired, and underwent develop- 

 ment by itself. This fauna, after a prolonged evolution in the northern 

 region, during which the Helderberg and Oriskany faunas were suc- 

 cessively occupying the interior, invaded the eastern interior sea. 

 Upon entrance, this fauna in part displaced and in part commingled 

 with the Oriskany fauna which then occupied the ground. Some 

 lingering representatives of the Helderberg fauna were probably also 

 present. This admixture of exotic and indigenous forms, evolving 

 together under the congenial conditions of the interior sea, developed 

 the later phases of the Onondaga fauna. 



The Southern Hamilton Fauna. 



The life which occupied the eastern portion of the interior sea 

 during the earlier part of the Hamilton epoch will be best understood 

 by calling again to mind the geographic changes which permitted the 

 entrance of a new factor from the south and controlled the develop- 

 ment of the whole. There appears to have been a warping of the sur- 

 face by which the land at the northeast was gently lifted and its rivers 

 rejuvenated, so that they bore much more silt into the basin than 

 before, and this, spreading widely over the bottom, changed the deposit 

 from limestone to shale in that part of the basin, while the formation 

 of limestone continued as before in the western portion. This warp- 

 ing movement may have been accompanied and abetted by climatic 

 and vegetal conditions favorable to silt wash. The warping appear? 



