144 GEOLOGY OF THE SECOND DISTRICT. 



CHAPTER VIII. 



Rocks composing the Taconic system ; order of superposition ; general strike and dip. — 

 Liability to mistake some of the slates and limestones for those which belong to other 

 systems. 



The number of rocks which compose the Taconic system is quite limited ; this is an 

 important feature, which is not to be lost sight of. It does not, however, follow that it is 

 necessarily thin ; on the contrary, it is remarkably thick, and hence becomes of more impor- 

 tance than appears from a simple statement of the number of rocks of which it is composed. 



As a whole, we find granular quartz, slate and limestone to form the entire system. But it 

 is to be remarked, that it is necessary to take cognizance of two kinds of slate and two of 

 limestone ; for although there are many points of resemblance in each of the two rocks respec- 

 tively, still their position and lithological diiferences, though small, require their separation. 

 The full enumeration is as follows : 



1. A coarse oranular limestone of various colors, which I have demoninated Sl^ckbridge limeskme, taking its name 

 from a well-known locality, one wliich has furnished to diiferent parts of the Union a large proportion of the white and 

 clouded marbles which have been so extensively employed for building and other purposes in construction. 



2. Granular quartz rock, generally fine-grained, in firm tough crystalline masses of a brown color, but sometimes white, 

 granular and friable. 



3. Slate, which for distinction I have denominated Magrwsiau slat^:, from its containing magnesia, a fact which is 

 distinctly indicated by the soft feel peculiar to rocks when this earth forms a constituent part. 



4. Sparnj limfstone, generally known as the sparry limerock. 



5. A slate, which I have named Taamic date, and which is found at the western base of the Taconic range. It lies 

 adjacent to the Lorrain or Hudson river shales, some varieties of which it resembles. In composition, it contains more 

 alumine and less magnesia than the magnesian slates. 



In addition to the above rocks, there is sometimes a slate of a dark color, and quite sili- 

 ceous, in the granular quartz. This appears less constant, and may be considered as a slaty 

 quartz, or variety of this rock. There are several deposits, important in themselves, which 

 strictly belong to this system : the hematitic iron ores, associated sometimes with carbonate 

 of iron, and the black o.xide of manganese. Subordinate to the rocks, we find milky quartz 

 and chlorite, with carbonate and oxide of iron in the magnesian slate. 



The following section, extending from Petersbugh, Rensselaer county, to Adams in Massa- 

 chusetts, embraces all the rocks in this system. Its direction is nearly east and west, or 

 perpendicular to the strike of the system over which it passes : 



