[WILLIAMS] FOSSIL FAUNAS OF THE ST. HELEN'S BRECCIAS 241 



Conclusions : 



It seems quite evident frc\m the critical study of the species tliat 

 neither of these St. He(len's faunas is to be' correlated exactly with any 

 one of the known faunas of New York or of the interior of the American 

 continent. Nor do they agree exactly with any of the more eastern 

 faunas of Maine, Quebec, New Brunswick or Nova Scotia. Nevertheless 

 a resemblance is found for the 1st fauna with the Oriskanian and for the 

 2nd fauna with the Helderbergian which they bea,r to no other knouTi 

 faunas. 



In correlating such unique faunas it must be borne in mind that the 

 causes for the modification of faunas are various. At the same point in 

 geologic time were living at different places in the same ocean basin deep 

 and shallow water faunas, adapted to warm and cold water, and those 

 differentiated by mingling of fresh and salt water. These different con- 

 ditions may be suddenly or slowly changed for any particular area of the 

 bottom. So that it is reasonable to look for considerable divergence in the 

 cojinposition of the faunas of a single geologic horizon. 



The wider study of the faunas indicates that we are here dealing 

 not only with a lot of arbitrarily distinct local faunas but faunas which 

 may be classified together into at least, two magnafaunas, the one re- 

 presented in the Helderberg and Oriskany beds of New York and the 

 other represented typically in the Hamilton of the same area. While tlie 

 succession of these two magnafaunas is clear in the American a,rea the 

 mingling of representatives of the two in the European Coblenzian 

 suggests that they were partly contemporaneous and that their differences 

 are to be accounted for rather by their evolution in two distinct and 

 distant basins, than by derivation of either from the other. 



It is quite probable that the first of these magnafaunas had its centre 

 of evolution and distribution in the North Atlantic in later Silurian time 

 and that the other was at the same time being evolved in the southern 

 seas. They came together in soine of their elements on the weistern 

 European continent in Lower Devonian time. On the other hand in the 

 intej-ior of America, they are sharply distinct until after the arrival of 

 Oriskany. 



The separation of these magnafaunas on the eastern border of the 

 American continent may be assumed to have been determined by prevail- 

 ing currents. 



'To explain the St. Helen's local faunas the hypothesis is offered 

 that ^the general diastrophic movement upward of the eastern bolder 

 province terminated the strictly marine conditions connecting with the 

 northern Atlantic basin at the geologic epoch represented by the erosion 



