102 DERANGEMENTS OF THE TACONIC SYSTEM 



VII. DERANGEMENTS OF THE TACONIC SYSTEM IN NEW-YORK, MASSACHU- 

 SETTS AND VERMONT. 



§ 1. Difficulty of distinguishing some rocks which conform in strike. 



The relations of the members of this system are without doubt preserved over the whole 

 extent comprised within the area of Rensselaer and Washington counties in New-York, 

 and Berkshire and Bennington counties in Massachusetts and Vermont. Here at least the 

 succession and parallelism of the rocks are maintained generally ; and in deciding what 

 rocks really belong to the system, we are not embarrassed by intruded masses. The geo- 

 logist may pass immediately over a succession of taconic rocks by and on the route of the 

 macadam road from Troy to East Bennington, a distance of thirty-five miles. The suc- 

 cession is uninterrupted, so far as intruded and plutonic rocks are concerned ; and therefore 

 all those masses or rocks over which he will pass in this easterly route, and which are 

 conformable to and succeed each other as represented in the table (page 63) , I conceive 

 to belong to the Taconic system, commencing at Troy with the Taconic slate, and termi- 

 nating in East Bennington with Granular quartz and the Stockbridge limestone. We end 

 here with the granite and gneiss of the Hoosic mountain. If this undisturbed condition 

 prevailed universally, there would probably have lieen but one opinion in regard to the 

 independence of this system. It was very natural, in the early days of geology, to regard 

 those rocks as identical which looked alike ; and hence when it was known that the talcose 

 slates of the Gneiss system differed but a fraction from those of the Taconic, it was to be 

 expected that they would all be placed upon the same list, especially wlien it was observed 

 that real primary schists ranged side by side with them in a few localities. An uninter- 

 rupted succession, however, does not prevail ; and it is my business, in this place, to give 

 some details of those interruptions which occur in New-York. 



It is well known that the range or strike of the Taconic rocks is nearly parallel with that 

 of the primary schists upon their eastern border, a fact which has had its influence in 

 observing the true age of the rocks under consideration. But this is not all : wherever, 

 from any cause, the lower rocks have been elevated, the ridges formed thereby lie also 

 parallel, and appear as a part of the system of rocks among which they range, unless 

 indeed the intruded rock is an unstratified one, as granite or trap. When gneiss or mica 

 slate forms a range, it is with extreme difficulty that we can persuade ourselves that the 

 talcose looking slates are not also parts of the Mica slate and Gneiss system. 



The great point of difficulty, therefore, in studying the Taconic system, is where the 

 members are respectively separated from each other by intervening rocks whose lithologicjil 

 characters closely resemble those of this system. This is particularly the case at those 

 points where the highland ridges of Orange, Rockland, Westchester and Dutchess counties 

 send up their spurs to the north. In conformity with this fact, I may state generally that 

 the highlands of the Hudson separate and divide the Taconic rocks as a whole. They are 



