2lS ROCHESTER ACADEMY OF SCIENCE. [Oct. 23, 



Following the Canadian period was an epoch called the Trenton, 

 during which the sea-bottom here was slowly sinking, thus permitting 

 the accumulation of a great limestone terrane. Organic growth and 

 deca)- during a vast time is represented by 954 feet of dark lime- 

 stone. Roughly estimating from the present growth of coral reefs, 

 this limestone of the Trenton would require 190,000 years. It might 

 have taken a much greater time. Plant and animal life was slowly 

 changing, and the oldest known fishes, the first vertebrates, are found 

 in this horizon (') in Colorado. 



The close of the Trenton and the beginning of a new epoch was 

 determined by such change in geographic features as to throw open 

 this region to wash of land, thus stopping the growth of lime- 

 producing organisms and the accumulation of limestone, and 

 producing instead a deposition of silt or clay. In Xew York termin- 

 ology, this epoch is known as the Hudson or Hudson-Utica (the 

 names indicating localities where the strata are found at the present 

 surface), and is represented here by 598 feet of dark-colored shales, 

 which required for their accumulation a vast duration. Tli^e Trenton 

 limestone terrane and the Hudson shale terrane, together repre- 

 senting the Trenton period, are both widely developed, eastward to 

 the Hudson river, and westward through the Mississippi region, 

 indicating that the marine conditions were widely uniform in those 

 early times, and that the source of silt supply, or the dry land, was 

 probably to the north. 



The close of the Trenton period is the end of the great age of 

 tiie Lower Silurian, which includes all the formations thus far consid- 

 ered. The transition is marked here by the change from silt to sand 

 deposit due to shallowing of the sea, but this locality was not lifted 

 entirely out of water as were areas in the east. The next formation 

 is 83 feet of gray and blue shaly sandstone belonging to the Oneida 

 or Oswego epoch and regarded as the base of the Medina, from the 

 fact of its local character. 



The Oneida epoch and all the subsequent time represented by 

 our strata are included in the Niagara period of the Upper Silurian 

 age. Hut the strata are various and important, and mark minor time 

 divisions. The blue-gray sands of the Oneida were soon buried 

 beneath the dark red sands of the Medina. Over 1,000 feet of this 

 sediment accumulated, of which the uppermost 100 feet shows in the 

 lower Genesee ravine. The Medina beds indicate wide-spread 



(31 Hull. Geol. Soc. Amer., Vol. 3, pp. 153-172. "Preliminary Notes on the Discovery of a 

 Vertebrate Fauna in Silurian (^Ordovician) Strata," By C. D. Walcott. 



