156 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
faults, and were it not for the cvicJcnctc of tht^sc incrusted boulders 
the same doul)ts might be cast on the nature of this contact. 
However, this evidence is decisive. 
Rensellaer Plateau. — Resting; upon the western flanks of the 
Taconics is a thick V)Ianket of coarse sediments which makes 
what would otherwise be an uneven country into a fairly level 
plateau. These sediments, the Rensellaer grits, are composed of 
materials of a heterogeneous character, and have long be(;n con- 
sidered to l)e of Medina age.^ The evidence for this view is 
largely circ\imstantial, it being supposed that these beds are the 
eastern continuation of the Medina sandstone. However, the 
Taconic Mountains were supposed to have been worn down by 
the beginning of Silurian time on the evidence of the contacts at 
Becraft Mountain, Rondout, and Shawangunk Mountain. It is 
difficult to understand how sediments of such unassorted ma- 
terial could have been derived from a peneplained surface. 
Clarke- has suggested that they may probably represent the 
eastward extension of the late Devonian sandstones of the 
Helderbergs, and this view is more likely to be correct than is 
the older and long-accepted one. 
Conclusion. — It is in this region that we find the first evidence 
of any weight in favor of Taconic folding, though in only one 
locality in an exposure some ten feet across, is the evidence un- 
questionable. Whether the unconformities at Rondout, and at 
the locahty next to be described, are chance occurrences, or 
whether we are to see in them indications of conditions that 
existed over a vastly wider area is a question which may not be 
satisfactorily answered for years to come. 
Shawangunk Mountain. 
No clearer case of an unconformity between the Ordovician 
(Normanskill) and the Silurian (Shawangunk) can be seen in 
eastern North America than that admirably exposed near 
Otisville, N. Y. Numerous excellent illustrations of it have 
appeared in text-books and in special articles.^ 
"Dale, T. N. Bull. U. S. Geol. Surv., no. 242, p. 42, 1904. 
^Clarke, J. M. Mem. N. Y. State Mus., no. 9, pt. 2, p. 159-161, 1909. 
^Schuchert, C. Silurian formations of southeastern New York, New 
Jersey, and Pennsylvania. Bull. Geol. Soe. Amer., vol. 27, p. 545, 1916; 
also (Clarke, J. M., Bull. N. Y. State Mus., no. 107, pi. A, 1907. 
