63 



The position of this bed is about seven feet below the base 

 of the lowest Trilobite bed, and its outcrop in the ravine is 

 almost opposite Mr. Avery's barn. 



The Transition Beds. With the Nautilus bed the base of 

 the Hamilton shales is reached. Underlying it are thirty 

 feet of shales which contain a mixed Hamilton and Marcellus 

 fauna, and they are therefore regarded as transition beds 

 between the Marcellus and Hamilton. The shales are 

 capped by a bed of somewhat arenaceous limestone, six 

 inches in thickness and very fossiliferous. This bed lies 

 immediately below the lower Pleurodictyum or Nautilus 

 bed, its position being about fifty feet below the base of the 

 Encrinal limestone. It forms the top of the fall in the 

 ravine just below the bridge on which the Lake-shore turn- 

 pike crosses. Here the bed is full of Orthoceras ( 0. exile 

 Hall) and of gastropods (Bellerophon leda Hall, Loxo- 

 netna hamiltoniae Hall and L. delphicola Hall). The small 

 productoid brachiopod with the truncated beak — Stropha- 

 losia truncata Hall — is the most abundant fossil in this 

 bed, and as it seldom occurs outside it in this region, it 

 becomes a convenient form from which to name the bed. 

 The name Strophalosia bed has been adopted for it,* this 

 name at the same time indicating the geological position of 

 the bed, since the Strophalosia truncata Hall is a character- 

 istic Marcellus fossil. f The Strophalosia bed appears in the 

 cliff both above and below the mouth of Avery's Creek. 

 Passing southward, we find, after crossing the mouth of a 

 second small ravine, which opens near Avery's Creek, that 

 the cliff has a height of only twenty feet or thereabouts, and 

 is entirely made up of the transition shales, which also form 

 the walls of the ravine of Avery's Creek below the falls. 

 Several layers of concretions occur in these shales, but fossils 



•Faunas of the Hamilton Group, etc. 



fThis is the bed lettered (a) on Plate V. of the Report on the Fourth Geological 

 District by Professor James Hall. The position of Avery's Creek is there indicated 

 by the depression marked " 13 miles from Black Rock." The bed is referred to on 

 pp. 190 and 191 of that report, where it is spoken of as the westward extension of 

 the "thick mass of sandy shale, so abounding in conchiferous molluska in the 

 eastern part of the State which in the central part is still in great force." 



