28 NEW YORK STATE MUSEUM 



or three of these outhers consist of red, flinty sandstone quite hke 

 that on the Theresa and Alexandria sheets. There are also asso- 

 ciated masses of very flinty conglomerate, full of pebbles of Gren- 

 ville quartzite, and quite like the conglomerate previously described, 

 except for their more excessive induration. These beds certainly 

 appear somewhat older than the Potsdam of the border belt. But 

 there is as yet no decisive evidence of any material difference 

 in age. 



No fossils have been noted in the Potsdam of the mapped area. 

 The nearest point at which we have collected them is at Clayton, 

 where Lingulella acuminata was found. 



Theresa Formation 



General statement. A series of " passage beds " of alternating 

 sandstone, calcareous sandstone and dolomite beds overlies the 

 Potsdam everywhere in the circum- Adirondack region. To these 

 beds we have been applying, for mapping purposes, the name of 

 the Theresa formation. In the eastern sections these beds have 

 large thickness, 150 to 200 feet, and are .followed by the Little Falls 

 dolomite, the three together forming the upper Cambrian (Ozark- 

 ian) series of northern New York. Deposition was seemingly con- 

 tinuous between these formations, and they grade into one another, 

 without sharp boundaries, so that their separation from one 

 another is largely a matter of convention, though they constitute 

 three contrasted, lithologic units. 



In mapping the Thousand Islands region we encountered diffi- 

 culties with this classification. The Potsdam was, as usual, fol- 

 lowed by a series of passage bed character, to which we gave the 

 name of Theresa, but no representative of the succeeding Little 

 Falls dolomite is present. In the lower half of the Theresa we 

 found Lingulella acuminata in several localities. In 

 the upper half, however, we did not find this fossil but did find in 

 several places a coiled gastropod and occasional cystid plates. 

 These Ulrich identified with forms found in the Tribes Hill lime- 

 stone of the Mohawk valley, a formation which there lies uncon- 

 formably on the Little Falls dolomite, and which Ulrich regards as 

 the lowest formation of the New York Beekmantown. The beds 

 containing these fossils were quite similar to the lower ones con- 

 taining the Lingulella and we were unable to detect any break 

 between the two, and hence mapped them together as a single 

 lithologic unit, the two together not exceeding 60 to 70 feet in 

 thickness. 



