Appalachian Geosyncline. 95 



partial cycles extending from the close of the Jurassic into the 

 early Tertiary.* If this be so, it still further destroys any real 

 relationship between the limits of the present outcrops and 

 the original limits of sedimentation, even in cases where 

 residuals still rise above the upwarped Cretaceous baselevel. 



Relation of Post- Ordovician Erosion to Original Limits of 



Devonian. 



A more or less widespread crust movement took place at the 

 end of the Ordovician. Uplift and folding occurred over the 

 Appalachians as far west as the eastern side of the Great 

 Y alley, as is shown by the unconformity at the base of the 

 Silurian and the fact that 'fragments of Ordovician formations 

 enter into the Silurian basal conglomerate. This has been 

 called the Taconic revolution from the region where the signif- 

 icance of the unconformity was first recognized. The Taconic 

 and Green Mountains still stand in Vermont at elevations in 

 the neighborhood of 4000 feet. The question arises, — -how 

 much of the metamorphism and folding of that region should 

 be ascribed to the close of the Ordovician, how much to later 

 movements? Their elevations, high above the Cretaceous 

 baselevel, have been commonly looked upon as an inheritance 

 from the Silurian. Did they contribute waste to the Devonian 

 sediments and form a barrier in Devonian times, or were the 

 Silurian mountains already leveled before the Middle Devonian 

 so that the Devonian waste had to come from farther regions 

 and may possibly have mantled the beveled folds ? 



The plateau, outlier of the Rensselaer Grit, shown in fig. 1, 

 goes far toward supplying an answer. In the first part of this 

 article this formation has been described and reasons advanced 

 for holding that it is more probably of Middle Devonian age, 

 though it was assigned by the older geologists to the Silurian 

 and by J. M. Clarke to the Upper Devonian. The grit rests 

 unconformably upon the Upper Ordovician schist and lies 

 immediately west of the Taconic range. The latter is com- 

 posed of these same schists and rises a thousand feet above the 

 surface of the plateau. The Rensselaer formation contains 

 beds of slate, others of grit, and still others of conglomerate. 

 Feldspar, gneiss, and quartzite pebbles are abundant. The 

 material of the plateau shows therefore that it was not largely 

 derived from the schists of the Taconic range which now over- 

 shadows it. Furthermore the Rensselaer grit is itself folded 

 and metamorphosed, indicating that it has once been somewhat 

 deeply buried and while so buried subjected to powerful earth 



*See Abstract, "The Piedmont Terraces of the northern Appalachians," 

 Bull. Geol. Soc. Am., 1913. Paper read Dec. 30, 1912. 



