522 NEW YORK STATE MUSEUM 



up again 40 rods south by a low ridge and forms the sides and 

 bottom of the channel for nearly J mile, showing about 5 feet 

 of the base of the uppermost layers and at the apex of the fold 

 a foot or two of the shaly layer beneath it. This is the most 

 southern exposure on Oak Orchard creek. Fossils obtained show 

 it to be the horizon of the Guelph dolomite. 



Rochester shale. Station 28. The falls at Shelby village. 

 The exposure here shows the lower silicious layers of the Lock- 

 port limestone of which about 30 feet are seen in the falls. 

 These beds are very close on the base of the limestone series 

 for at a short distance farther down the stream is an outcrop of 

 the upper Rochester shale bearing a 1 foot silicious bed in the 

 middle of the bluff, and just below on the face of the escarp- 

 ment which is indicated by the 600 foot contour line of the 

 topographic map is a clean cut exposure of this shale. 



CUnton beds. Station 29. No further outcrop is seen along 

 the course of the creek till the southern outskirts of Medina 

 village are reached where (station 30) is an outcrop on the creek 

 bank £ mile north of the dam, exposing crystalline crinoidal 

 limestone with well defined Clinton fossils. 



Medina sandstone. A few rods farther down (station 31) the 

 creek exposes a white greenish coarse sandstone of the upper 

 Medina, and from there north Medina sandstone exposures are 

 of frequent occurrence throughout the region. From these ob- 

 servations we have the means of identifying with approximate 

 accuracy the contact lines of the Medina-Clinton, Clinton-Roch- 

 ester and Rochester-Lockport beds. 



Relation of the Oak Orchard swamp to these rock formations. A 

 map of sufficient size such as the topographic sheets of Albiou 

 and Medina indicating the extent and distribution of the swamp 

 areas in this region, shows that one can not ascribe to any 

 lithologic differences in these formations a sufficient cause for 

 the present area of these swamps. The marsh along the course 

 of Black creek between Byron and Bergen doubtless lies on the 

 Salina shales or is underlain by a pavement of Lockport dolo- 

 mite where those shales have been removed. But we observe 



