FIFTH REPORT OF THE DIRECTOR I908 



31 



and Devonic time, no rocks of this age being determinable. Profes- 

 sor Emerson regards all the rocks above these mentioned as Car- 

 bonic coextensive with the Worcester and Mansfield coals. 



These conclusions give evidence enough of an old land barrier 

 bounding a trough of Devonic waters in which the metamorphosed 

 beds of Bernardston at least were deposited. The rest may have 

 been removed by erosion, but in eastern New York between the 

 Hudson and the Massachusetts line and in the direction of the 

 Devonic rocks of Bernardston lies an extensive sheet of coarse 

 clastic material known as the Rensselaer grit which at this point 

 requires brief attention. 



Rensselaer grit. Rensselaer and Columbia counties, New York, 

 lying east of the Hudson river and in the general direction of 

 continuity between the Helclerberg-Catskill escarpment and the 

 Bernardston Devonic outcrops of the Connecticut valley, are ex- 

 tensively mantled by heavy arenaceous deposits lying uncon form- 

 ably on the unfolded Cambric and Lower Siluric strata beneath. 

 The character and distribution of this rock was clearly outlined by 

 Lieutenant Mather in his report on the first geological district 

 (1843) anc t ft was regarded by him as equivalent in age with the 

 Shawangunk grit of Ulster and Orange counties on the west of the 

 river. 



The early geologists held the Shawangunk grit to be an eastern 

 representation of the Oneida grit of central New York and this 

 conception has been quite generally promulgated. Mr T. Nelson 

 Dale has been one of the latest investigators of this region and has 

 acquired an intimate knowledge of the stratigraphic relations of 

 this terrane to the unconformable rocks beneath and we owe to 

 him the conclusion that the upfolding of the lower and upper 

 terranes pertains to different dates, the former to the Taconic and 

 the latter to the Postdevonic or Carbonic movement winch also 

 produced the more southerly synclines now represented by Becraft 

 mountain, Columbia county. Mr Dale has correlated the Rens- 

 selaer grit with the entire Oneida-Medina sedimentation of eastern 

 New York. In recent investigations carried on by C. A. Hart- 

 nagel [see Mus. Bui. 107. 1907. p. 5 1 j it is shown with approxi- 

 mate conclusiveness that in the typical sections of central New 

 York the Oneida conglomerate is not a formational unit but 

 actually lies within the Medina sandstones; that, further, the Shaw- 

 angunk grit, on stratigraphic evidence alone, is of an age much 

 later than the Medina formation and being overlain by rocks of 

 Postsalina age is presumably the eastern representation of Salina 



