154 



west part of Wrentham, as already stated, the Huronian 

 granite has been extravasated through schists that appear to be 

 Montalban. 



The fact that our Montalban strata are farther removed than 

 the Huronian from the axis of this region, as represented by the 

 Naugus Head series, is, I am persuaded, of some value as an 

 indication of their more recent formation. The rocks of the 

 Naugus Head series and the granite to the south of Boston, 

 are not only, as I conceive, in the stratigraphic axis of this 

 region, but they unquestionably represent the centre or axis of 

 disturbance for a large territory. We have seen how the dis- 

 turbance of the Huronian beds gradually diminishes from Boston 

 harbor north-westerly, and this gradation is continued without 

 interruption across the entire breadth of the Montalban to the 

 argillites of the Nashua Valley. The line connecting Hing- 

 ham and Shirley, or, what is the same thing, the line joining 

 the oldest Huronian and the newest Montalban, begins in the 

 most and ends in the least disturbed rocks of these two systems, 

 and there is a regular gradation between them. In fine, it 

 must be in the experience of all observers in Eastern Massachu- 

 setts, that the Montalban terranes exhibit on the whole vastly 

 less disturbance than the Huronian ; and that, as a rule, this 

 disturbance diminishes as we recede from the Huronian border. 

 Distinctly and evenly stratified rocks are the rule in the Mon- 

 talban formation, and the exception in the Huronian. 



STRATIGRAPHY OF THE NASHUA AND MERRIMAC VALLEYS. 



Since the days of the elder Hitchcock it has been known in 

 a general way that the Nashua Valley is a synclinal ; but the 

 careful studies of Mr. Burbank in the middle portion of this 

 basin have greatly advanced our knowledge of the relations of 

 the rocks, and furnished the key to a better notion of the struc- 

 ture of that entire region than has heretofore existed. The 

 principal facts bearing on this problem are as follows : Be- 

 tween the western boundary of the Huronian formation and the 

 mica slates of the Nashua Valley, in the northern part of Mid- 



