62 NEW YORK STATE MUSEUM 



the southern Adirondack region could not have been completelv 

 submerged at the close of the Lower Silurian, much less so at the 

 close of the Trenton. Here again the evidence is rather opposed 

 to that on the north side of the region, as will be shortly shown. 



Unconformity at the base of the Trenton 



The great thickness of the Trenton formation at Trenton Falls, 

 and its rapid diminution in thickness eastward across the area of 

 the map have already been noted, together with the variation in 

 thickness of the Lowville down to complete absence, the presence 

 of the Black river limestone only here and there, and the fact that 

 the passage beds between the Beekmantown and Lowville are not 

 always present. Vanuxem, Darton and Prosser have all pub- 

 lished valuable data along this same line, derived from the 

 Mohawk valley to the eastward. The most significant section i« 

 that at Canajoharie, described b}- all three observers as showing 

 a distinct, though slight, erosion unconformity between the Beek- 

 mantown and Trenton [see pi. 10, and compare with pi. 5]. 

 Prosser measured but IT feet of Trenton here, and this seems to 

 represent only the upper beds of the formation, while both the 

 Lowville and Black river are wholly wanting. At Sprakers, 3 

 miles farther east, Prosser's section shows again but 17 feet of 

 Trenton, with no sign of the Lowville and Black river. Nothing 

 is said about an unconformity at this point and apparently the 

 actual contact is not exposed. 



Eastward from Sprakers the Trenton slowly thickens, and the 

 Lowville and Black river limestones reappear. Prosser's numer- 

 ous and accurately measured sections, published in the 15th 

 Annual Keport of the State Geologist, show that all three 

 are usually present, though occasionally either the Lowville or 

 the Black river is lacking, and the combined formation does not 

 regain any special thickness, being always under 50 feet. 



From these facts it is evident that the rather steady, progres- 

 sive subsiding movement which characterized the region during 

 Beekmantown deposition, and which resulted in the rather con- 

 stant thickness of that formation of from 450 to 550 feet through- 

 out the Mohawk valley (except where thinned by overlap), was 



