142 



strange if some future geologist should regard a part of this 

 range as mica slate." Substantially the same remark is 

 repeated in connection with the gneiss of Plymouth and Bristol 

 Counties. The gneiss of the great central area between the 

 Nashua and Connecticut Rivers is seldom very micaceous ; yet, 

 toward its western border^ it encloses several considerable 

 bands of mica slate, and appears to pass gradually into the 

 mica slate of the Connecticut Valley ; and along the eastern 

 margin we find the gneiss of Mt. Wachusett, and the range of 

 highlands of wliich this summit is the culminating point, rich 

 in mica, in many places at least, and occasionally passing into 

 mica slate. The mica slates here considered are everywhere 

 intercalated in the gneiss, and are essentially a part of it; but 

 the important bands of mica slate, already described, in the 

 Nashua and Merrimac Valleys, are undoubtedly newer than the 

 gneiss ; and yet, it is believed, exhibit at many points a gradual 

 passage downwards into that rock. The lower and older parts 

 of the mica slate pass insensibly into the upper beds of gneiss, 

 on which they repose. Of the mica slates of the first or 

 eastern band, Mr. Burbank says :^ " They are very variable in 

 mineralogical character ; but all become coarser toward the 

 south-eastern border, and finally pass into gneiss." And in 

 another place ^ he speaks of the petrologic relations of the 

 Middlesex County gneiss as follows : ' ' This belt of gneiss is 

 bounded on the north-west by the slates of the Nashua and 

 Merrimac Valleys, which apparently rest conformably upon it. 

 Moreover, there appears to be a gradual transition in passing 

 westward, from the coarsely crystalline gneiss, through mica 

 and hornblende schists, to the thin-bedded clay slates like the 

 roofing slate of Lancaster. . . . From a series of careful 

 observations on these rocks, I am convinced that the slates 

 above referred to cannot be separated from the underlying 

 gneiss, hnt J^orni with it a continuous series." Toward the 



* Sketch of the Geology of the Nashua Valley accompanying my report on the 

 Geological Map of Mass., p. 45. 

 *Proc. Boat. Soc. Nat. Hist., xiv., 190. 



