143 



western margin of the third belt of mica slate, where we ap- 

 proach the Wachusett gneiss, the same gradual transition is ob- 

 served, and is, perhaps, even more marked. The stratigraphic 

 connection here is not so complete as on the east ; for the slates 

 are not seen to repose on the gneiss of the Wachusett range, 

 and are probably separated from it by an extensive fault. 

 But the general fact remains that, in spite of a complete 

 stratigraphic inversion, the slate of this belt becomes coarser 

 and more decidedly micaceous as we proceed from its newer 

 to its older beds. Prof. Hitchcock, speaking of the mica 

 slate of the Nashua Valley as a whole, as one belt, says : * 

 ' * As we, approach the east and west sides of this range the 

 characters of the mica slate beeome more decided ; and in this 

 slate of apparently greater antiquity the veins and protruding 

 masses of granite are more numerous." 



Looking now in the opposite direction we see the mica 

 slate passing upwards into the argillite. The evidence of a 

 gradual transition appears, on the whole, even more conclusive 

 here than along the lower limits of the mica slate, where it 

 meets the gneiss. The views of Mr. Burbank on this point 

 are clearly expressed in what I have above quoted ; and Prof. 

 Hitchcock undoubtedly regarded this mica slate as a sort of 

 transitional rock between the gneiss below and the argillite 

 above. Considerable portions of the mica slate, especially 

 toward its upper border, might be properly called argillite ; and 

 much of the rock marked on the map as argillite is more or less 

 micaceous. This gradation is apparent at most points where 

 the two rocks approach each other and are not drift covered, 

 but is perhaps best shown along the western border of the 

 argillite, in Lunenburg, Leominster, and Sterling ; where, ac- 

 cording to Mr. Burbank, it is well-nigh perfect. The extensive 

 drift deposits of this region, however, render it impossible to 

 prove the absence of any break whatever along this line ; while 

 in the Harvard conglomerate we seem to have, notwithstanding 

 the evidence to the contrary, a strong indication of at least a 



1 Final Report on the Geol. of Mass., p. 598. 



