no NEW YORK STATE MUSEUM 



Cambrian beds are all in minor folds, as are certainly the several isolated 

 Ordovician areas and the large central ramifying ones. And there is no 

 evidence of such a great overthrust as at Burlington and Saint Albans. But 

 Mr Walcott does find evidence of an overthrust on Bald mountain, in the 

 town of Greenwich, in Washington county, New York, west of the slate belt. 

 The exposures described near North Granville (page 292) also indicate 

 reverse faulting. Yet the course of the Cambro-Ordovician boundary along 

 the western edge of the slate belt, particularly northeast and southwest of 

 North Granville, New York, and in Benson and Hubbardton, Vermont, and 

 again in the townships of Hartford, Argyle, and Hebron, New York, is 

 hardly consistent with the existence there of a great longitudinal overthrust, 

 nor do the vertical relations of the Ordovician and Cambrian outcrops favor 

 such a construction. If such a thrust plane separates the two formations, it 

 must be a folded thrust plane, which is not an ordinary probability. 



It would appear, therefore, that the great mid-Silurian orogenic movement 

 which, in northern Vermont, operating upon rigid beds, found relief in a 

 great overthrust, at the south, near the slate belt, operating upon beds which 

 were more plastic, compressed them into minute folds. In either case the 

 compression at the south found relief chiefly in folding, and only here and 

 there, as about North Granville and at Bald mountain, in faulting. But evi- 

 dences of faulting may be found west of the area here mapped. 



In 1901 (page 555ff.) the writer explained the inverted order of 

 the formations observed by him in the neighborhood of Albany 

 by an overturned fold which changed into an overthrust fault, 

 whereby the Georgian became overthrust upon the underturned 

 wing. This view was also reached by Dale in the Geology of the 

 Hudson Valley between the Hoosic and the Kinderhook (1904, 

 page 38). He states: "That the relations on the west side of 

 the Cambrian belt are not only those of unconformable deposi- 

 tion but of more or less continuous overthrust is rendered some- 

 what probable from the situation of the overthrust near Schodack 

 Landing and of that at Bald mountain in Greenwich, 47 miles 

 north-northeast of the former, as well as from the general direc- 

 tion of the Cambro-Hudson boundary between them. . . . For 

 these reasons it is assumed that in conseguence of a westwardly 

 overturned fold, which is frequently ruptured, the Lower Cam- 

 brian here usually overlies the Hudson, that is, the Trenton or 

 middle part of the Ordovician." 



The subsequent work of the writer seems to indicate that this 

 view as to the origin of the overthrust of the western margin in 

 which he and Dale agree, is the one nearest the truth. We see 

 possible evidence of this hypothesis in the belt of Bald Mountain 

 limestone of Beekmantown age, outcropping below the Georgian 



