136 STRATIGRAPHICAL GEOLOGY. 



of the porphyritic gneiss over the whole area between the Sandwich and 

 Pemigewasset River ranges ; and, as the boundary Hne between tliem 

 conforms to the strike of neither, it is Hkely that they are not divided by 

 a fault, but the irregular edge is due to unequal erosion by atmospheric 

 agents. Nevertheless, the line is straight enough to have been deter- 

 mined by a fracture; but even in that event the two formations must 

 have been unconformable to each other. The very easterly directions of 

 the strata at H. K. Hill's and V. G. Durgin's would seem to indicate 

 the passage of the porphyritic rock directly underneath the Montalban 

 gneisses along the line of strike. A similar course is probable for the 

 very interesting outcrop of the older rock in the Pemigewasset river 

 along the boundary between Thornton and Campton. Upon Section 

 VII will be found a representation of the situation of this rocky blanket 

 upon the porphyritic floor. 



Little search has been made for the mineral andalusite in this range. 

 There is, however, an outcrop near E. Cone's, at the crossing of the road 

 by a tributary of Mad river, near a cemetery, showing either this mineral 

 or kyanite sparsely scattered through the ledge. The dip is 60° S. 42° 

 E. It is rare to find ledges in this part of Thornton, on account of the 

 abundant loose materials spread everywhere over the surface. Passing 

 southerly beyond the limits of the White Mountain district, this forma- 

 tion assumes a much greater importance, and covers an immense area. 



Eruptive rocks. There is much to be learned from the study of the 

 various eruptive rocks connected with this group. In the absence of a 

 proper study of their lithological characters, little attention has yet been 

 devoted to them. Hence they must be referred to again in another part 

 of the report in the department of mineralogy. 



One of the most important areas of eruptive granites intimately asso- 

 ciated with this group lies between the Portland & Ogdensburg Railroad, 

 north of the Notch, and the base of the presidential ranges, including 

 Mt. Deception. The great contrast in the topographical features of this 

 granitic region with those of the harder schists may be well apjDreciated 

 by reference to the view of the Mt. Washington range from the P^abyan 

 turnpike. Vol. I, p. 392. The whole of the foreground is granitic, and of 

 two varieties, different from those in the Pemigewasset series. That 

 from the Mt. Deception range resembles the Concord gneiss, save that 



