96 J. Barrell — Upper Devonian Delta of the 



pressures and high temperatures. It is, however, on the western 

 margin of these orogenic activities. These relations of attitude, 

 of composition, and of internal structure show that the present 

 Taconic region was lowlying in the Devonian, no longer a 

 mountain barrier, and that a large part of the strong folding, 

 thrusting, and metamorphism which now mark the Taconic 

 and Green Mountain region was imposed after the Rensselaer 

 grit was deposited, probably in the closing revolution of the 

 Paleozoic. 



In New Jersey also the Green Pond Conglomerate of 

 Silurian age rests unconformably upon Ordovician limestone 

 and pre-Cambrian gneiss. Its pebbles, except at the immediate 

 base, are water-worn pebbles of white quartz and not local 

 accumulations from an unreduced region. Much erosion had 

 taken place between the close of the Ordovician and the 

 beginning of Silurian deposition. 



Further, Lower Devonian quartzites and fossiliferous lime- 

 stones are found immediately east of the Green Mountains in 

 northern Massachusetts over a region doubtless affected by the 

 Taconic folding. Clearly then we must conclude that the 

 mountains of the Taconic Revolution did not endure to supply 

 the Upper Devonian sediments. They no longer rose as in- 

 superable barriers and the sediments were more or less free to 

 transgress their eroded structures. 



Influence of Late Paleozoic Cycles of Erosion. 



Here will be considered from the standpoint of theory the 

 possible effects of the erosion cycles which followed the deposi- 

 tion of the Upper Devonian but preceded the Permian fold- 

 ing- 



The original marginal parts of a formation are the least 

 down warped. They may, in fact, if terrestial in origin, have 

 accumulated at considerable elevations above sea level. Even 

 if not originally above sea level a slight upwarp of the margins 

 of the basin might completely destroy them when the central 

 and thicker parts would still remain. While a deposit exists 

 as an unconsolidated surface formation it is particularly sensi- 

 tive to even minor changes of level. The most widely extended 

 formations are the ones least protected by overlying materials 

 and most constantly subjected to erosion through later time, so 

 that, as Gilbert has pointed out, the evidence of the greatest 

 past transgressions of the sea becomes most completely obliter- 

 ated.* After such an erosion of marginal strata let the sea 

 again invade the land and a disconformity is the result, in 

 which the time interval marked by the hiatus represents some- 

 continental Problems, Geol. Soc. Am. Bull., iv, 187-190, 1893. 



