140 II. P. Gushing — Lower Paleozoic Rocks of JVew York. 



siderable one than that between the black and dove limestones 

 was suggested by the limited distribution of the 7 foot tier as 

 compared with that of the chert beds beneath, the latter 

 accompanying the Lowville all the way up the Black River 

 valley and also across into Ontario, while the 7 foot tier was 

 restricted to the immediate vicinity of Watertown. Ulrich 

 also urged that, in many localities without the State where he 

 had studied the sections, the lower break was bridged by 

 deposit, so that the dove limestone Lowville graded upward 

 into the cherty beds ; and that the sharp lithologic difference 

 between the two in the Watertown region was local, rather 

 than the general rule. He conceded that at Watertown the 

 natural method for areal mapping purposes was to class the 

 two black limestones, the chert beds and the 7 foot tier, together 

 as we had done. But he emphasized the fact that this was 

 not the usual rule, that in many districts no separation of the 

 chert beds from the Lowville was possible, and that the name 

 Lowville limestone was capable of vastly more extensive appli- 

 cation as a formational name were the chert beds included in 

 the formation at the type locality. 



Ruedemann, Ulrich and Cushing had also, during 1907, 

 1908 and 1909, carefully studied the beds, latterly called Black 

 River, in many localities on all sides of the Adirondack region. 

 These beds are mostly thin, and often of patchy distribution, 

 especially in the Mohawk valley, and we found that they were 

 of quite various age, representing the thin, shoreward edges of 

 embayment deposits, with repeated and quite local oscilla- 

 tions of level. Plainly a considerable time interval was repre- 

 sented, and the term Black River was one of very loose 

 application. 



In the final reports of the four geologists of the early survey 

 (1842) the name Black River limestone was somewhat vari- 

 ously used by the different men, though the statements of 

 Vanuxem, Mather and Hall in regard to it do not greatly vary. 

 They make it include the interval between the Calciferous and 

 the Trenton. Vanuxem heads his chapter on the group as 

 follows : 



"Black River limestone. Synonyms — Birdseye limestone, 

 Mohawk limestone, base of the Trenton limestone, as used in 

 the reports of the Third District. Black marble of Isle la 

 Motte, Seven-foot-tier, and Chazy limestone of Dr. Emmons, 

 the latter mass connecting the Birdseye with the Calciferous 

 sandrock 'proper.' 1 ''* 



The formation was chiefly confined to the Second and Third 

 Districts, and Mather and Hall, in their reports, apparently 

 simply followed Vanuxem. 



*Geol. 3dDist., p. 38. 



