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disturbance ; it is nearly cut off, as Mr. Burbank has observed, 

 by considerable areas of granite ; and the axial plane appears 

 to be shifted or warped so as to dip toward the east. 



That the two bands of mica slate extending north-easterly 

 from Harvard and Ayer form a synclinal is apparent not 

 only from their convergent inclination, but also from the fact 

 that the underlying gneiss in each case shares the dip 

 of the mica slate. But that the two sides of the synclinal 

 do not meet is evident from the fact that a broad belt of 

 granitoid gneiss, an expansion of the Worcester and Dracut 

 range, lies between them. In other words, the bottom of the 

 synclinal has been entirely removed, exposing the underlying 

 granite and gneiss. To raise this immense mass of the subjacent 

 rocks to their present position and leave the band of mica slate 

 on either side unremoved by denudation requires two faults, 

 one on each side of the valley between the granite and 

 mica slate, with the downthrow in each case on the side 

 toward the latter rock. But in Harvard we already have two 

 such faults bounding the granite ; and we have only to conceive 

 these to diverge slightly and extend north-easterly along the 

 margins of the Merrimac synclinal, and the structure of the 

 whole region becomes clear immediately. PI. 3, fig. 3, repre- 

 sents a general section across the Merrimac synclinal from 

 Westford to Dunstable. This section fully accounts for the 

 absence of argillite from the Merrimac synclinal, that rock 

 having been raised, with the most of the mica slate, far above 

 the present surface, and subsequently entirely swept away by 

 denuding agents. It must have required an uplift of 10,000 

 or 15,000 feet to raise the lowermost stratum of the mica slate 

 above the present surface. 



The mica slate on the north side of this synclinal cannot 

 be certainly traced much beyond the Merrimac ; and it seems to 

 become quite narrow before reaching that point. This is readily 

 accounted for by supposing that the northern line of fracture 

 swept so far to the north as to make it possible for the present 

 surface of denudation to cut below the mica slate. The granite 



