PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 437 



which Ave could wish. Ascending to a greater geological height — 

 the base of the carboniferous, by means of outliers of the con- 

 glomerate, we are led to the unavoidable inference that full thirty 

 miles of the northern edge of the coal scries have been almost 

 entirely swept awa}'; and not only this, but full, seven hundred 

 feet in depth of the strata lying immediately beneath, over a very 

 large portion of the country. 



The most remarkable example we know of, of the efiect of this 

 species of denudation, is on the east, northeast and north aspect 

 of the Catskill mountains, viewed at a distance from either of these 

 directions, they appear as if upheaved : but viewed from the 

 westward, they appear but as the termination, in very bold preci- 

 pices, of the great devonian plateau, which extends from Georgia 

 north-eastward, and terminates so abruptly a little west of the 

 Hudson river. They are mountains, not because of upheaval, but 

 because the basal edges of the strata composing their bulk, have 

 been cut away upon their sides fronting the horizon, from which 

 they appear as mountains. 



By what means shall we attempt to estimate the amount of 

 denudation their strata have sufiered in the line of their strike? 

 There are two methods — one to prolong their trend, until lines 

 drawn according to the rate of the diminution in the thickness of 

 the strata from the maxima to the minima, or uaitil the lines meet 

 in an angle. In Pennsylvania, these strata are 11,000 feet; in the 

 Catskill, only 5,000 feet; now, following this same rate of dimi- 

 nution, before the strata could thin out, they would reach the Adi- 

 rondacs in Essex county, north of the Mohawk; and east of the 

 Hudson, we find outliers of the lower portion as far northeast as 

 the center of Washington county. In Canada, the devonian is 

 found, still farther north, as fav as Collingwood bay. From these 

 rude and immature data, we can imao-ine that the feather edfje of 

 the devonian once rested on the Adirondacs and the Tacouic 

 mountains. 



But it is south of this devonian plateau, in Pennsylvania, 

 Maryland and Virginia, that we are the most sure of our data. 

 Here, the coal is present on the west, and in patches on the north, 

 east, and in the centre of the great chasm, scooped out of the 

 eastern and middle portions of Pennsylvania. Besides these rem- 

 nants, there are numerous upthrows of silurian strata, which con- 

 firm us in our deductions. The whole thickness of the carbonif- 

 erous, 3,000 feet, devonian, 7,000 feet, and upper silurian, 2,000 



