112 NEW YORK STATE MUSEUM 



consist of Georgian rocks. Likewise at Grant hollow, where the 

 writer found the Deep Kill graptolite fauna in the bottom of the 

 gorge, the tops of the hills on both sides of the gorge are fossili- 

 ferous Georgian. The latter occurrence is of great interest because 

 it connects the Ordovicic shale belt, west of the overthrust line, 

 with the large Mt Rafinesque-Rice Mountain '' Outlier." If the 

 Georgian rocks overlie the Beekmantown graptolite shale at Grant 

 hollow, it can be inferred that this outlier is really a " Fenster," or 

 a portion of the Ordovicic rocks underlying here the Georgian mass, 

 but exposed by erosion. 



There is considerable and quite conclusive evidence that the 

 thrust plane is irregular in its hade, through folding; for while 

 the thrust plane is very slightly inclined at Bald mountain and 

 the Moses kill, it is steep east of Willard mountain, and in the 

 neighborhood of Troy. That these differences are due to folding 

 of a character transversal to the general northeast strike of the 

 beds is indicated by the fact that where the hade is steep, the 

 Georgian rocks descend deeper than where it is flat, as in the val- 

 ley of the Batten kill, these regions corresponding to depressions 

 or synclines. 



There is considerable evidence extant of folding of the entire 

 region long after the Green Mountain revolution, marking the 

 Ordovicic-Siluric boundary, and which is considered responsible 

 for the principal folding and overthrusting of this region. Such 

 later folding is shown by the folded condition of the Rensselaer 

 grit, that is probably of Upper Devonic age, and the remnants 

 of the folded and overthrust Devonic limestones still found along 

 the Hudson river. We have here especially in mind the Kings- 

 ton region, the folded and overthrust structure of which has been 

 so well worked out by Van Ingen and Clark.^ These authors 

 have shown there the presence of a practically horizontal over- 

 thrust plane, the thrust of which comes from the east and which 

 is possibly a manifestation of the same force that pushed the 

 Georgian rocks over the Ordovicic rocks on the other side of the 

 Hudson. 



The question whether the overthrust at the western edge of 

 the Cambric area recognized by both Doctor Dale and the writer, 

 reaches so far back east that the large Georgian area is all over- 



1 Gilbert van Ingen and P. Edwin Clark. Disturbed Fossiliferous Rocks 

 in the Vicinity of Rondout, N. Y. N. Y. State Mus. Bui. 69, p. 1176. 1903. 



