Topography of Gneiss. 



389 



State, and in breadth several miles, the characters of the gneiss 

 are greatly obscured by the iron aspect which the rocks have assumed 

 in consequence of the decomposition of pyrites. The same appear- 

 ance is frequent in other parts of the range. 



Granitic gneiss abounds in various parts of this deposit : but ra- 

 ther more I think in the southern than in the northern part of the 

 State. In the west part of Charlton, for instance, and so occasionally 

 all the distance to Brookfield, one is often at a loss whether the rock 

 be gneiss or granite. In Uxbridge, in the range east of the mica 

 slate, the same variety abounds ; and still more frequently on the east 

 side of the Blackstone in Mendon. 



That range of the Worcester county gneiss just referred to, which 

 extends northeasterly into Middlesex county, possesses some peculiar 

 characters. In another place 1 shall attempt to show, from the dip 

 and direction of the strata, that it belongs to a different system of strat- 

 ification from the gneiss west of Worcester valley. But I refer now 

 to other peculiarities. One is, that it contains numerous beds of lime- 

 stone, which are entirely wanting in the western range. Another is, 

 that it passes so frequently into mica slate ; the two rocks often 

 alternating, and indeed, in some places, the slate predominating. In- 

 deed, it would not be strange if some future geologist should regard 

 a part of this range as mica slate. A third pecularity is, that it 

 abounds, especially towards its northeastern extremity, in veins and 

 protruding masses of granite. 



I have found it very difficult to determine the exact eastern limits 

 of the gneiss range under consideration. I mean the line of its junc- 

 tion with the granite. Much of the gneiss near that line is granitic, 

 and of course difficult to be distinguished from granite. In some 

 places the beds and veins of granite increase in number and size as 

 we go easterly, until at length the gneiss occupies only a small pro- 

 portion of the surface. Near the junction of the two rocks, also, di- 

 luvium is very abundant ; which increases the difficulty of fixing 

 their limits. 



New Bedford Gneiss. 



On the first edition of the map, I represented a deposit of gneiss 

 extending from Rochester to Little Compton : but subsequent exam- 

 ination has rendered it doubtful to my mind, whether any thing more 

 than small insulated patches of this rock are to be found west of New 

 Bedford. Gneiss of a peculiar character, does, indeed, appear in 



