W. B. D wight— Potsdam strata near Poughkeepsie. 125 



be expressed remain to be determined. The number of these 

 constants will doubtless be diminished by future generaliza- 

 tions and the application to lithology thereby simplified. 



Since the rate at which heat is evolved in a solidifying lava 

 is the greatest possible, the rate at which it cools, other things 

 being equal, must be a minimum. This fact must have an im- 

 portant influence on the flow of eruptive rocks and probably 

 accounts in part for the power which they possess of penetrat- 

 ing the tiniest cracks as dikes. 



San Francisco Office U. S. Geol. Survey, Nov., 1885. 



Art. XII. — Recent Explorations in the Wappinger Valley Lime- 

 stone of Dutchess County, K Y.; by William B. Dwig-ht. 



No. 5. — Discovery of fossiliferous Potsdam Strata at Poughkeep- 

 sie } JV. Y (Illustrated by a map, Plate VI.)* 



The presence of rocks of the Potsdam group in association 

 with the limestones and shales of Dutchess County, N. Y., has 

 long been suspected on stratigraphic grounds, but until the 

 present time this fact has never been proved by positive paleon- 

 tological evidence. 



At the bases of Fishkill and Stissing Mountains, a thick 

 stratum of quartzyte is found between the underlying Archean 

 of those mountains, and the overlying limestones, now known 

 to be respectively Calciferous and Trenton. This quartzyte 

 shows planes of stratification, and is conformable to the over- 

 lying strata of limestone. Obviously, by its stratigraphic 

 position, it appears to represent the Potsdam group, and this 

 assignment has been made for it, in the localities mentioned, 

 by Professor Mather, Professor J. D. Dana and others. 



Notwithstanding that considerable search has been made, no 

 fossils have yet been found in this quartzyte stratum, by 

 which this reasonable hypothesis might be established. I am 

 not aware that any geologist has heretofore found reason to 

 suspect the presence of a Primordial stratum among the lime- 

 stones of the region, and certainly I have had no such expecta- 

 tions myself. The observations made during the last few years 

 in the Wappinger Valley (or "Barnegat") limestones, have 

 definitely proved them to be composed extensively and continu- 

 ously of conformable strata of the Calciferous and Trenton 

 groups. In carrying out the work, in which I have for several 

 years been engaged, of preparing a detailed stratigraphical 

 chart, on April 25th of the present year, to my great surprise, 



* Read before the American Association for the Advancement of Science, at 

 Ann Arbor. 



