28 NEW YORK STATE MUSEUM j 



or three of these outhers consist of red, flinty sandstone quite like j 

 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 



i 



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 j 

 the Theresa formation. In the eastern sections these beds have j 

 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- I 

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

 without sharp boundaries, so that their separation from one 

 another is largely a matter of convention, though they constitute i 

 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 i 

 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 w^e were unable to detect any break 

 between the two, and hence mapped them together as a single 1 

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

 thickness. 



