TACONIC SYSTEM. 141 



terniptcd up to the Old redsandstone. But if we commence an examination at the foot of the 

 Hoosic mountain, which is gneiss, we pass over a series totally different from those of which 

 we have just been speaking, and among which the Potsdam sandstone does not appear, 

 neither a limestone which can be referred to those of the Champlain group, or slate or shale 

 which can be recognized as belonging to the New-York system. If we are correct in this 

 conclusion ; if the Taconic rocks differ as much as has been represented from the Primary, 

 and also from the Transition series, then it appears necessary that we should adopt views at 

 least somewhat analogous to those expressed in the preceding pages. 



Much that is useful in the discrimination of rocks, may be learned from the mode in which 

 minerals are connected with them. As we do not expect to find tourmaline, garnet or stau- 

 rotide in an aqueous deposit, so when they do occur in their appropriate rocks, they have a 

 uniform connection with them ; they belong to the mass, and are contemporaneous with it ; it 

 is rare that any cavities appear around them, but they are closely invested on all sides by 

 the materials of which the rock is composed. On the contrary, in aqueous rocks, minerals 

 occur in preexisting cavities, and they are usually composed of substances more or less solu- 

 ble under one or more conditions. 



In the Taconic system, there are two or three instances in which minerals do occur as in 

 the Primary rocks ; thus, octahedral magnetic iron appears in the magnesian slates, and ex- 

 tremely iine needle-form schorl in a siliceous slate. A few instances of this kind, however, 

 ought not to overthrow the views here expressed ; they may serve to weaken, but they are to 

 be set over against numerous instances which have a contrary bearing. We are to remember 

 that these rocks belong to the earliest deposits ; that they have been exposed to agencies 

 which, if not of a different character, were yet more intense than those at present in opera- 

 tion ; or that they have been exposed to molecular action aided by caloric, which must result 

 in a new arrangement of the particles of which they are composed. 



General Strike and Dip. 



Confining the observations to the Taconic range, that portion especially adjacent to the 

 eastern borders of Columbia, Rensselaer and Washington counties, and the western part of 

 Berkshire in Massachusetts, I have found the strike to vary from 10° W. of N. to 10° E. of 

 N. If we direct our attention to the country one hundred and fifty or two hundred miles dis- 

 tant, we shall find but little variation of the strike from a north and south direction. The 

 variation, therefore, seems to be quite local, and will be found confined to short distances. 

 The several rocks which have been enumerated coincide also in the direction of their strike, 

 thereby affording evidence of their belonging to but one system. 



The dip is very uniformly east, sometimes steeply so, but averaging only 30° or 55°. It 

 is proper to remark, that the dip of the Primary rocks upon the east have apparently the same 

 direction, but it is really much greater ; a fact which shows that there is no passage of the 

 Taconic rocks beneath the Primary. This dip is regarded as a remarkable fact, and one 

 which, in the view of some geologists, required a complicated movement; a movement 



