141 



granite is found for the most part in just the position indi- 

 cated. When we come to consider the stratigraphic relations 

 of these rocks it will be shown that the oldest gneisses of 

 Eastern Massachusetts occur mainly along the eastern border 

 of the Middlesex County area, and in Bristol and Plymouth 

 Counties ; and here it is, as we have already seen, that the grani- 

 toid character of the gneiss is most perfectly and extensively 

 developed. It is in these regions, too, that it presents, 

 on the whole, the steepest dips, the greatest disturbance, and 

 the most thoroughly crystalline appearance. I am aware that 

 in other parts, also, of the former area, and in the Nashua and 

 Blackstone Valleys, the gneiss sometimes exhibits a decidedly 

 granitoid aspect. In most instances, however, these apparent 

 disagreements with the view here proposed will disappear if we 

 are careful to exclude all the endogenous granite, and to dis- 

 tinguish between gneisses that are merely coarse, and those 

 that are truly crystalline ; while in other cases I think it can 

 be shown, or at least may be reasonably inferred, that the 

 granitoid patches in question are portions of the lowest and 

 oldest beds of gneiss, which have attained their present positions 

 through the agency of either faults or folds of the strata. The 

 Worcester and Dracut range of granite and granitoid gneiss is 

 the most considerable of these outlying areas ; and in another 

 place (post, pp. 156—158) I shall attempt to prove that this 

 belt has been brought to the surface by two profound fractures 

 of the crust, and probably represents a relatively ancient and 

 deep-seated portion of the gneiss. 



Of the passage of the gneiss into mica slate we have 

 abundant evidence. Much of the gneiss of the broad area in 

 Middlesex County and the eastern part of Worcester County is 

 highly micaceous, and includes numerous limited beds of distinct 

 mica slate (marked as gneiss on the map) . Professor Hitchcock, 

 in describing this gneiss, says l : " It passes frequently into mica 

 slate ; the two rocks often alternating, and indeed, in some 

 places, the slate predominating. Indeed, it would not be 



1 Final Report on the Geol. of Mass., p. 629. 



