1864.] ON THE GEOLOGY OF EASTERN NEW YORK. 369 



Bparry limestones of the Quebec group appear in the valleys, 

 while the hills are of slate. Proceeding thence westward towards 

 the river, only the lower portions of the Quebec group are met 

 with, until we come upon the rocks of the Hudson River group. 



Washington Mountain is also of slate, flanked by limestone, all 

 of the Quebec group, and is probably synclinal in structure. The 

 valley to the south of the mountain exhibits limestones, apparently 

 alternating with slates. Columbia and Dutchess counties appear 

 to be mainly occupied by the shales of the Quebec group, with 

 broad exposures of its limestones, until we approach the river to 

 the westward, when the shales of the Hudson River group are met 

 with, extending a considerable distance below the city of Hudson. 



From Fishkill the explorers proceeded to Coldspring, crossing 

 what Mather called the Mattewan granite, but which they found 

 to be an altered sandstone. Soon after this they came upon the 

 great gneiss formation of the Highlands of the Hudson, which 

 continues beyond Peekskill. They failed to find the sandstone 

 described by Mather as coming out at this place ; nor was anything 

 representing the Potsdam sandstone detected in approaching the 

 Highlands from Fishkill, nor elsewhere along their northern limits. 

 Near to Peekskill, in the valley of the creek, was found a low 

 ridge of black slate, supposed to belong to the Quebec group, and 

 a similar slate was observed alono- the north side of the Hiahland 

 range, not far from the gneiss. The gneiss of the Highlands 

 presents all the aspects and characteristics of that of the Lauren- 

 tian system, as seen in northern New York and in Canada. 



Further examinations are necessary to determine the extension 

 to the north-east of the Laurentian rocks of the Highlands, and 

 also the succession of strata to the south-east of them. The recoo-- 

 nition of the Sillery and of the Quebec group in this region are great 

 and important facts for its geology, and not less so the identification 

 with the Laurentian system of the gneissic district of the High, 

 lands, to which the interesting mineral region of Orange county 

 and the adjacent parts of New Jersey doubtless belongs. This 

 conclusion, although opposed to the views of Mather and Rogers, 

 who looked upon the crystalline rocks of the latter region as 

 altered Lower Silurian strata, is in accordance with the older 

 observations of Vanuxem and Keating, and with the more recent 

 ones of Professor Cook, according to all of whom the gneiss and 

 crystalline limestones of Orange County and of New Jersey 

 underlie unconformably the Lower Silurian strata. t. s. h. 



