﻿GEOLOGY OF THOUSAND ISLANDS REGION 93 



Ulrich has recently made the important discovery of an uncon- 

 formity between these beds and Division A, and we coincide in be- 

 lieving that this division is properly to be classed with the beds below 

 rather than with the Beekmantown. The Beekmantown that is 

 thinly present in the district here reported upon is not of the Cham- 

 plain type, but of the Mohawk valley type, lithologically and f aunally 

 quite like the beds at Little Falls and thence eastward through the 

 Mohawk valley, which have heretofore been called the " fucoidal 

 beds," and which we are proposing to call the Tribes Hill formation. 

 This Beekmantown did not come into this northern district from the 

 east but from the south, and so far as we know did not extend on 

 eastward. But in passing to the eastward, beyond the limits of the 

 region here mapped, Beekmantown beds begin to appear above the 

 Theresa, and in our belief are unconformable, though this has not 

 yet been demonstrated. Also in our belief this Beekmantown is not 

 representative of the lower portion of the formation but of the upper 

 portion,' and we must go yet farther east to find the lower beds com- 

 ing in; while the thin edge of Tribes Hill Beekmantown in our dis- 

 trict here is lowest Beekmantown. Following a condition of uplift 

 Beekmantown submergence seems to have commenced fairly simul- 

 taneously on the east, west and south sides of the Adirondack region. 

 Submergence on the west was quickly followed by emergence due to 

 a general eastward tilting of the region, so that at Little Falls and 

 about Theresa only a slight thickness of the very lowest Beekman- 

 town was laid down, the Tribes Hill formation. This formation 

 steadily thickens to the eastward, along the Mohawk valley, though 

 apparently representing nothing but the lowermost Beekmantown. 

 The chief area of Beekmantown sedimentation in New York was the 

 Champlain valley trough and its prolongation southward. Along 

 with the steady subsidence in that trough seems to have gone a sub- 

 sidence of the St Lawrence trough which, like the previous Potsdam 

 subsidence, seems to have commenced at the east and worked west- 

 ward ; so that, in that trough, the lowest Beekmantown is absent, and 

 steadily higher beds are at the base going West. The extreme west- 

 ward reach of this Beekmantown depression of the St Lawrence 

 trough seems never to have reached the Theresa district, where the 

 only Beekmantown represented is the thin base of the Tribes Hill 

 formation of the Mohawk Beekmantown type. Until the Beekman- 

 town on the north side of the Adirondacks has received more 

 thorough study, this view of Beekmantown conditions in the St Law- 

 rence trough can not be regarded as based on sufficient evidence, 

 though evidence on the other three sides of the Adirondack region 



