GEOLOGY OF THE WHITE MOUNTAIN DISTRICT. 1 35 



72° W. At the saw-, shingle, and clapboard mill on the Woodstock line, 

 gneiss dips 80° south-easterly. South of Hatch hill, on the road east, 

 there are Montalban schists resembling those found farther north. 



The immediate vicinity of the Pemigewasset is occupied by porphyritic 

 gneiss noticed on page 102. There is an interesting section between 

 Welch mountain and the river at West Thornton, represented in Fig. 12. 

 At the westward is the Pemigewasset river, with its alluvium nearly to 

 H. K. Hill's house. The underlying rock is the porj^hyritic gneiss, with 

 a very high easterly dip, nearly vertical, — the other part of the anticlinal 

 axis existing on the west bank of the river, and not represented. The 

 strike is N. 38° E. The hill is the continuation southerly of the east 

 water-shed of the river, and not a rock boundary. As soon as we come 

 to Mill brook the rock changes to the Montalban schists. At W. Graves's 

 they dip 85° south-easterly, and are much contorted. A similar position 

 is noted all the way to Welch mountain, but especially at the school- 

 house east of J. P. Sanborn's. Cone mountain is composed of the same 

 rock. This designation is derived from the name of a person, and not 

 from any resemblance to the well-known geometrical solid. At E. F, 

 Elkins's the rocks are concealed ; but we pass up the steep slope of 

 Welch mountain, which is composed of Albany granite in broad plates 

 dipping twenty degrees westerly, and supposed to overlie the Montalban 

 group. The lower granite was not observed at this point. 



This section illustrates, first, the mutual relations of the three rocks 

 named. Supposing them to lie in their natural positions, one sees clearly 

 that the porphyritic gneiss is the oldest formation, and possesses an 

 anticlinal struxture, while the same uj^per schists appear on both sides 

 of the axis. The granite assumes the overlying, slightly inclined position 

 argued for this rock almost universally in New Hampshire. Second, 

 there is such a divergence in the strike of the porphyritic and Montalban 

 groups in Thornton that one may be justified in inferring that there is an 

 unconformity between them. The strike all through the central forma- 

 tion of the section is north-easterly. The course of the boundary between 

 them is N. 10° W. The most common strike of the porphyritic gneiss is 

 a little east of north, but at V. G. Durgin's the longer axes of the crystals 

 run nearly east and west. Assuming the correctness of these statements, 

 it may be inferred that the Montalban schists lie upon the upturned edges 



