GEOLOGY OF THE BROADALBIN QUADRANGLE 33 



these authors the old Beekmantown or Calciferous of the Mohawk 

 valley should be divided into a lower mass of Little Falls dolomite 

 (Upper Cambric) and an upper mass of Tribes Hill limestone 

 (Ordovicic), the two being separated by a distinct unconformity. 



This Tribes Hill limestone, which attains a thickness of i68 feet 

 at Cranesville on the Mohawk river, is almost, if not entirely, 

 absent from the Broadalbin sheet. Just off the map, and close to 

 the Haines quarry, one and one-half miles north of Mayfield, there 

 are a few feet of bluish-gray, fine-grained, thin-bedded, fucoidal 

 layers clearly underlying the Black River-Trenton limestone near 

 the quarry. These fucoidal layers the writer regards as Tribes 

 Hill. Near Kegg's quarry, one-half mile east of Cranberry Creek, 

 the Lowville is almost in contact with the dolomite. The quarry 

 itself was filled with water at the time of the writer's visit but speci- 

 mens of the gasteropod (Ophileta) so common to the Tribes Hill 

 formation were found in limestone fragments about the quarry and 

 this strongly suggests the presence of a touch of the Tribes Hill 

 here. Along the creek, two miles east of Broadalbin, the Lowville 

 is almost in contact with the dolomite so that very little, if any, 

 Tribes Hill comes in here. In other parts of the quadrangle there 

 is no positive evidence of the occurrence of Tribes Hill limestone. 



The unconformity between the Little Falls and the Tribes Hill 

 in the Mohawk valley is of considerable consequence because it 

 shows that the Cambric period in this part of the State, at least, 

 closed with the land above water and undergoing erosion. Again, 

 Ulrich and Cushing have shown that the summit of the Tribes Hill 

 in the Mohawk valley is also marked by an eroded surface thus 

 producing an unconformity between the Tribes Hill and the over- 

 lying Black River-Trenton. Thus the Chazy is absent from this 

 part of the State and during Chazy time the land must have been 

 above water and suffering erosion. This fact is especially signifi- 

 cant with reference to the Broadalbin sheet because the Tribes Hill, 

 though doubtless originally present in considerable amount, has 

 been almost, if not completely, removed during this erosion period 

 so that with the inauguration of Black River-Trenton time those 

 limestones were deposited directly upon the Little Falls dolomite. 



BLACK RIVER-TRENTON LIMESTONES 



As a result of recent changes in the nomenclature of the Lower 

 Paleozoic formations of northern New York, the term " Black 

 River" is no longer applied to a single limestone formation but 



