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PI. 3, fig. 1, represents a general section of the Nashua 

 synclinal, beginning in the central part of Bolton, and ex- 

 tending through Lancaster and Sterling to Mt. Wachusett 

 in Princeton. It shows the probable relations of the 

 rocks, the positions and direction of the faults mentioned, 

 and especially the way. in which the second fault has dupli- 

 cated the mica slate on the east side of the syncline. To 

 the north of this section the line formed by the intersection of 

 this fault and the present surface follows the western side 

 of the strip of argillite and conglomerate to its termination 

 near Harvard centre. This isolated mass of argillite is the key 

 to the structure of the whole eastern side of the valley. It is 

 clearly a small slice cut off by the granite from the great mass 

 of argillite on the west, with which it is unquestionably identi- 

 cal, lithologically. In its present position, conformably over- 

 lying the firsthand of mica slate, it affords an accurate measure 

 of the horizontal displacement resulting from the fault, and 

 proves conclusively the stratigraphic equivalence of the first 

 and second bands of mica slate. Of course, the assertion that 

 the conglomerate, in its normal position, conformably underlies 

 the great mass of argillite unavoidably raises the question as to 

 why it is not found in that relation in other parts of the valley. 

 Wliy is not this parting of conglomerate found in every section 

 across the common boundary of the argillite and mica slate? 

 The answer is, that, in the first place, the conglomerate is thin 

 and may be of local extent ; secondly, these rocks are so exten- 

 sively drift-covered that the precise boundaries are rarely seen ; 

 and, thirdly, evidence is not wholly wanting of the occurrence 

 of the conglomerate at other points in the valley. Mr. Bur- 

 bank has observed, in the drift near the city of Worcester, 

 boulders of conglomerate identical with the conglomerate of 

 Harvard ; and yet, if transported by glacial action, these bould- 

 ers could not have been derived from the Harvard belt, but 

 must have come from some point as far west as the western 

 border of the argillite. Along the geological boundary just 

 mentioned, in Lunenburg and Leominster, the same observer 



