CANANDAIGUA AND NAPLES QUADRANGLES 15 



region where there has been but little deep stream-cutting. 

 Owing to their rigidity they are highly jointed and rhoniboidal, 

 triangular and diamond-shaped slabs being characteristic of all 

 surface exposures. It is difficult to estimate the thickness of this 

 bed but it appears, from comparison of outcrops here with the 

 total thickness afforded by the Livonia salt shaft section where it 

 was 43 feet with a slight tendency to increase eastward, to be 

 about 50 feet. The actual contact of these beds with the underly- 

 ing Onondaga limestone has not been observed, but the lowest out- 

 crop of the formation on this quadrangle appears on the west 

 side of the fill on the New York Central Eailroad, just north of 

 the cut near Padelford. The higher beds are well exposed in this 

 same cut where they are densely black shales with some thin 

 limestone layers. The same beds appear along Mud creek about 

 a mile south of Mertensia. 



The distinctive character of this shale as an initial part of the 

 beds which have heretofore generally been assigned to the Mar- 

 cellus stage, is its uniformly bituminous nature and consequent 

 dark color and its very small proportion of lime content except 

 in the thin calcareous beds themselves. 



Stafford limestone 



The group of strata which have customarily been incorporated 

 within tlie general term Marcellus shale embraced an interesting 

 limestone layer, the presence of which was early noted by Pro- 

 fessor Hall and which was termed by the writer some years ago 

 Stafford limestone^ on account of its high development at Stafford 

 in Genesee county. This is a dark chocolate and somewhat nodu- 

 lar limestone, very hard when fresh but breaking easily into 

 angular fragments on exposure. We have shown in various pub- 

 lications that this formation extends eastward with a diminish- 

 ing thickness and we know that its last surface appearance is 

 along Flint creek in the southwestern part of the town of Phelps, 

 ' Ontario co. Though not exposed to the eastward it is evident 

 that the formation in slight thickness (it has a thickness of about 

 8 inches in Phelps) occurs as a thinning wedge through this area, 



