159 



probably right in insisting that the complication of this region 

 is largely due to ill-defined, vertical movements of partially 

 plastic materials, rather than to well-marked faults in rocks 

 essentially rigid. ^ . 



In passing through Harvard and Littleton, the first band of 

 mica slate sweeps around to a north-easterly course, parallel 

 with the Merrimac Yalley ; and beyond Harvard the dip is to 

 the north-west. The second band of this rock, reappearing in the 

 north part of Harvard, widens rapidly northward through Ayer 

 and ,Groton, until it meets the gneiss in southern Dunstable ; 

 here it divides into two branches, the principal of which con- 

 tinues due north, with westerly dips, into New Hampshire, 

 while the other strikes a north-east course, running parallel with 

 the first band to and probably across the Merrimac. The two 

 parellel bands of mica slate here indicated as diverging in a 

 north-east direction from the main Nashua synclinal, and ex- 

 tending, the one from Harvard to Lowell, and the other from 

 Ayer to Tyngsboro', dip toward each other, forming, as Mr. 

 Burbank has pointed out, a well-marked synclinal. In short, 

 the first and second bands of mica slate are one and the same 

 bed, which, from Worcester to Harvard, has a nearly uniform 

 dip to the west ; but north of Ayer it becomes warped, and 

 instead of forming, as it were, one side of a synclinal it now 

 forms three sides; i.e., in the place of a single synclinal, we 

 have two synclinals and an intermediate anticlinal ; the great 

 synclinal of the Nashua Valley sending off a branch to the 

 north-east. The main or parent synclinal continues northward 

 to the New Hampshire boundary, and then bends to the north- 

 east, and runs through southern New Hampshire, following the 

 north- v\^est side of a long and narrow area of Montalban gneiss, 

 and possibly older rocks, which begins in Dunstable. This is 

 an anticlinal ridge, having a synclinal on either side of it. Be- 

 fore leaving Massachusetts the main synclinal exhibits great 



' Concerning the structure of the region about the Harvard conglomerate, the view 

 here proposed bears some resemblance to that previously advanced by Mr. Burbaak, 

 and wherever the two views coincide his is entitled to priority. 



