vi F\ REPOPwT OF PROGRESS. E. W. CLAY POLE. 



His cross-sections I have redrawn to make the vertical 

 scale correspond with the horizontal scale ; and to make 

 thf out ciop-belts conform in width to the general scale of 

 thicknesses given on page 32, and in Plate V, page 34. The 

 plication of the underground rocks of Perry county is so 

 greal that its rock-beds come to the surface at all angles, 

 from 1° to 90°; and are even overturned in the south-east 

 corner of the county where the older formations overlie 

 those which were deposited upon them. (See the Index of 

 Dips on pages 404, 405.) 



In drawing cross-sections it is easy to exaggerate the 

 amount of plication. To avoid such exaggeration, it is 

 needful to execute them on one and the same scale, vertical 

 and horizontal. If this be done, and the formation- thick- 

 nesses be only approximately correct, then the formation- 

 dips will come right in spite of minute local variations. 

 If, for example, the Oriskany outcrop Xo. VII and the 

 Dellville sandstone outcrop at the base of No. IX be well 

 marked on one of the township maps w T here the dip is all 

 one way — and if there be 5600 feet of Marcellus, Hamilton, 

 Genessee, Portage, Chemung, and passage beds of No. 

 VIII — it follows that a cross-section on a dip of 33° will rep- 

 resent the structure with a close approximation to the 

 truth. 



1i' there should be a local thickening of the deposits of 

 VIII to 6000 feet, or a local thinning down to 5000, then, in- 

 stead of using for the cross-section a dip of 33°, one must 

 use 36 or 28°. It is plain, therefore, that very considerable 

 errors in estimating the thicknesses of the formations will 

 produce a hardly appreciable effect upon the character of 

 t li,> cross-sections if they be drawn to a true scale both 

 vertical and horizontal, provided fixed points of reference 

 have been secured upon the map. 



1 have constructed six sections on Plate IV, page 32, 

 partly to exhibit this truth, and partly to show that the 

 apparently excessive plication of Perry county is not of an 

 extraordinary kind, nor so great as that of the Anthracite 

 coiil logins. 



I had also in view the illustration which it affords of a 



