338 



H. P. CUSHINi 



NORTHUMBERLAND VOLCANIC PLUG 



can not be discussed intelligibly without a brief consideration of the rock 

 formations of the neighborhood. 



THE SECTION 



In the region in which the knob stands are two sharply contrasted sets 

 of early Paleozoic formations, apparently deposits of two separate troughs, 

 the one indigenous to the district and the other overthrust into it from 

 the east. The knob stands within the eastern overthrust set of forma- 

 tions and the boundary between the two lies some five miles west of it. 

 The western set are comparatively undisturbed and consist of the ordi- 

 nary formations of the Champlain trough (Chazy basin), though the 

 representation of these is very imperfect. The entire Beekmantown and 

 Chazy groups and most of the Black Eiver are not found in the section 

 which comprises, in ascending order, the Potsdam sandstone and Little 

 Falls dolomite of Upper Cambrian (Ozarkian) age, the Amsterdam lime- 

 stone of Upper Black Eiver age, and the Canajoharie shale of Trenton age. 



The eastern set of formations are the Lower Cambrian (Georgian), in 

 great thickness ; the Dictyonema (Schaghticoke) shale, of extreme Upper 

 Cambrian or lowermost Ordovician age ; the Deepkill shale, of Beekman- 

 town age; the Bald Mountain limestone, also of Beekmantown age, and 

 the Xormanskill and Snake Hill shales, of Chazy or Black Eiver and of 

 Trenton age. They are greatly disturbed and have been overthrust in 

 sections, so that the upper shales reach farthest west ; the Bald Mountain 

 limestone has been overthrust on them, and the Georgian overthrust on 

 the Bald Mountain. These overthrust beds thin out to disappearance to 

 the west and must rest on the deposits of the Chazy basin. Therefore it 

 would be possible for the limestone inclusions in the knob to have come 

 from either set of formations. At the knob Xormanskill shales are the 

 surface rocks. Quite likely these rest on Bald Mountain limestone, fol- 

 lowed beneath by the Canajoharie shale, Amsterdam limestone, Little 

 Falls dolomite, and so on — the regular succession of the rocks of the 

 Chazy basin. 



Now, unless some formation occurs here underground which does not 

 outcrop at the surface, some representative of the Beekmantown or Chazy 

 formations of the usual Champlain section, the only formation of the dis- 

 trict which could possibly have furnished such inclusions is the Bald 

 Mountain limestone. The only limestone in the formations of the Chazy 

 basin is the Amsterdam, which is thin and lies directly above the great 

 thickness of Little Falls dolomite. It is unthinkable that igneous rock, 

 rising through this series of formations, could have brought up such an 

 abundance of fragments from the Amsterdam and not one from the great 





