in the Narragansett Basin. 57 



Evidence of ttvo orographic periods. — Another point of 

 interest is the evidence that two periods of orographic upheaval 

 have taken place in the area : The first, at the close of, or dur- 

 ing. Pennsylvanian time, was accompanied by the invasion of 

 an extensive granite batholith ; the second was sufficient to 

 develop steeply dipping folds and intense compression effects, 

 but there is no evidence that it was accompanied by granitic 

 intrusion. It seems, from the writers' acquaintance with the 

 surrounding region, that the first upheaval acted generally in 

 an east-west direction, accounting for the present prevailing 

 north-south strikes of strata and of schistosity, whereas the 

 second acted for the most part in a nearly north-south direction, 

 developing the folds with nearly east-west axes. Further study 

 of the folds in the southeast part of the Narragansett Basin 

 may require a modification of the last part of this statement. 

 The earlier upheaval was probably very widespread, whereas 

 the second one, based on correlations which follow, extended 

 over a relatively limited area of the present land surface, which 

 embraces the present Narragansett, Norfolk County, and Bos- 

 ton basins, but whose outer limits are not known. The char- 

 acteristic alteration and abundance of slickensided fractures in 

 the Dedham granite are most marked in the areas adjacent to 

 these basins, and are the same in composition and character as 

 those which characterize the more highly sheared and com- 

 pressed exposures of the conglomerates in the basins. As these 

 conglomerates contain Dedham granite pebbles, the alteration, 

 both of the granite and of the conglomerates, is believed to 

 have been developed during the second orographic period. 



Pondville Conglomerate. * 



The Pondville conglomerate,* or Pondville group as Wood- 

 worthf called it, consists of gray to whitish basal arkose and 

 suprabasal conglomerate beds. It is limited to the northern 

 border of the Narragansett Basin and to the southeastern bor- 

 der of the Norfolk County Basin. It grades upward into the 

 Wamsutta formation and rests unconformably upon biotite 

 granite, separating the latter from the overlying Wamsutta 

 formation, save at a few places where Wamsutta beds rests 

 directly upon the granite. The type section at Pondville in 

 the Norfolk Basin shows granite passing by almost insensible 

 gradations into arkose, which is directly overlain by alternat- 

 ing beds of arkose and grits with vein-quartz pebbles 

 and occasional nodules of biotite granite. Above these is an 

 alternating succession of quartz-pebble conglomerates and red 



*This name has been adopted bv the U. S. Geol. Survey. 

 fOp. cit., pp. 134-141. 



