g6 ROCHESTER ACADEMY <JF SCIENCE. [.Ull. II, 



Scio, occurs on the highest hills " {//>/(/., p. 410). The exposure 

 last mentioned is probably the one called Little Genesee by Pro- 

 fessor Williams and regarded as equivalent to the Clean conglom- 

 erate. 

 i. Professor Hall wrote : "The summit of the Portage group, on the 

 (lenesee river is less than 1200 feet above tide water. * * * * 

 The highest hills toward the south part of the State are scarcely 

 less than 2500 feet above tide water, showing a difference of ele- 

 vation between the two groups of 1300 feet. Allowing for undu- 

 lations, which render the dip irregular, the whole thickness is 

 above 1500 feet " (Oeol. N. \'., Pt. I\', p. 260). Since Professor 

 Hall did not distinguish between the flat and the angular pebble 

 conglomerates it is difficult to decide which of these conglomer- 

 ates should be taken for the top of his Chemung. From certain 

 statements we are inclined to think that the 1500' was intended to 

 represent the thickness of the rocks from the top of the Portage 

 to the base of the Olean conglomerate. A comparison of Pro- 

 fessor Hall's " Section from the mouth of the Genesee river to 

 Instantur, Penn." and the "Vertical section showing the relative 

 thickness of the different rocks" (2d .\n. Kept. Fourth Geol. Dist, 

 N. Y., 1838) seems to indicate that then he considered the rocks 

 from an "argillaceous iron ore" stratum near Wellsville, N. Y., 

 down to "olive shale and sandstone " near Portage as 1750' in 

 thickness. These rocks correspond pretty closely with the limits 

 of what Professor Hall later defined as the Chemung group of 

 this region ; but the thickness would be overestimated if it were 

 based on the supposition that the dip amounted " to 50 or 60 feet 

 in the mile " {J hid., p. 291). 



Professor Williams' investigations seem to assign a thickness 

 of between 1500' and 1600' to the rocks between the Wolf creek 

 conglomerate and the top of the Portage sandstones. The alti- 

 tude of the top of the Portage sandstone at Portage Falls is about 

 1200' (Bull. U. S. Geol. Surv., No. 41, p. 52) ; altitude of \\'olf 

 creek conglomerate west of West Clarksville about 1950' {Ibid. 

 p. 86); hence 1950' — i2oo'=75o'. The distance between the 

 two places is about 33 miles which with a dip of 25' to the mile 

 {Ibid., p. 103) would equal 825'. Then 75o' + 825'==i575' for the 

 thickness of that series of rocks. 



Mr. Harris, by computation and the record of the Jamestown 

 well, makes the thickness of the rocks from the Panama conglom- 



