386 



Scientific Geology. 



the Hoosac Mountain range ; two in the central parts of the state ; 

 and a fourth in the vicinity of New Bedford, in Plymouth and Bris- 

 tol counties. 



Hoosac Mountain Range.. 



Under mica slate I have already given a general description of the 

 situation of this gneiss, and the reasons that lead me to suspect that 

 it passes laterally, or in the direction of the strata, into mica slate- 

 Throughout nearly the whole extent of Litchfield county, in Connect- 

 icut, this gneiss range is most distinctly characterised. As we pro- 

 ceed northerly into Massachusetts, its characters become less decided. 

 The feldspar is less abundant, and the mica more so; and hornblende 

 frequently abounds in it: so that viewed on a small scale, itmay often be 

 regarded as mica slate. Along the eastern slope of Hoosac Moun- 

 tain, the rock becomes sooner converted into mica slate than along the 

 western slope. On this latter side, indeed, distinct gneiss continues near- 

 ly across the State, as may be seen on the map ; and I am by no means sure 

 but careful research may trace it entirely across the State; so as to con- 

 nect it with the gneiss that appears in the lower part of Vermont, 

 along the eastern talus of the Green Mountains. At any rate, that 

 Vermont gneiss appears distinctly characterised in the southwest part 

 of Whitingham, near the beds of limestone, as shown on the map ; 

 and from thence I have traced it as far north as the Somerset iron 

 mine. Here then we have two wedge-shaped ranges of gneiss, with 

 their acute angles towards each other, while the space between them 

 is occupied by mica slate and talcose slate ; and sometimes we find 

 these slates, for a limited space, passing into gneiss. There is certainly 

 then some probability in the supposition that all these schistose rocks 

 may belong to one formation ; and that the varieties resulted from 

 local peculiarities in the mode of formation. 



In passing from Becket to Lee, we cross strata of decided gneiss, 

 till within three or four miles of Lee, when the rock contains a con- 

 siderable proportion of hornblende, and at length becomes decided 

 hornblende slate. Still nearer to Lee, the hornblende is replaced by 

 green augite, and augitic gneiss hence results. Within two miles of 

 Lee, we strike upon limestone ; which often contains a mixture of au- 

 gite ; and this mineral, being decomposed at the surface, yet projecting 

 beyond the limestone, the whole rock exhibits a brown and very 

 rough and irregular aspect ; exceedingly like similar compounds at 

 the lime quarries in Bolton, Boxborough, Littleton, &c. — This au- 



