131 



in the Illinois basin during the time of deposition of the lowest 

 of the Grand Tower strata. Of these forms Michelinia stylopora 

 and a species of Zaphrentis occurred in the underlying Ori- 

 skany (Clear Creek) beds. Out of twenty-eight species of 

 fossils associated with the corals in these lower sandstone 

 layers, thirteen, or nearly 47 per cent, were present in the Up- 

 per Oriskany (Clear Creek) strata of Illinois. Seven out of 

 these thirteen species have not been reported from the Jeffer- 

 sonville limestone, while the other six are forms having a wide 

 distribution. Of the remaining fifteen species from the sand- 

 stone layers, ten are not reported by Kindle from the Jeffer- 

 sonville beds of Indiana. The other five forms have a consid- 

 erable vertical, and a wide geographical, range. 



The small number of these coral species in the lower Grand 

 Tower strata, and the absence of the greater number (60 per 

 cent) of their associates from the Indiana area, would pre- 

 clude the assumption that the Jeffersonville limestone fauna 

 could have been derived from this source. 



It may also be seen from the faunal lists in the foregoing 

 section that the cephalopod element in the fauna is limited to 

 the upper portion of the Grand Tower beds, although in New 

 York the cephalopods become conspicuous in the early Onon- 

 daga strata ; also that the characteristic Onondaga fish fauna 

 of Ohio never reached this Illinois basin. 



From the above considerations it is assumed that one arm 

 of the Onondaga sea advanced on the continent from the south, 

 as in the preceding Oriskany time, reaching as far north as 

 Jackson county, Illinois. Other sea incursions are thought to 

 have come in from the north and east, bringing the 

 corals and the cephalopods into the interior of the continent. 

 This northern interior sea is thought to have been separated 

 by a land barrier from the southern basin in which the Grand 

 Tower limestone was deposited, until the middle of Onon- 

 daga time. Not until near this same time is it thought that 

 the northern sea, bringing the JefTersonville fauna, extended 

 as far south as southern Indiana. Not until somewhat later 

 still was the barrier which separated the northern and southern 



