GEOLOGY CF THE MERRIMACK DISTRICT. 53 1 



of later origin, notice of their limits must be deferred. Opportunity may- 

 be afforded for the perfection of our general sections before the final 

 completion of our explorations, in which case the reader will find in the 

 atlas further details of structure in this ancient formation than appear 

 from our descriptions. 



We have not yet identified the Bethlehem gneiss in the eastern part 

 of the Merrimack district. In the western part of the district three areas 

 of a fine-grained variety are separated from the more characteristic rock, 

 and delineated upon the map ; and it is likely this kind of material, if not 

 the other, might be found east of the porphyritic ridge. This distinction 

 was made after the conclusion of all field work, so that it has not been 

 practicable, as yet, to apply it to the eastern region. There is also a 

 scarcity of hornblende in the Merrimack valley, the rock made use of in 

 the Connecticut valley to mark the limits of the Bethlehem areas. 



2. Lake Gneiss. 



These rocks occupy about the same territorial area as the main porphy- 

 ritic range just described. There are three principal lines of its occur- 

 rence, divided into many subordinate or isolated areas, which have been 

 separated on account of concealment by overlying later formations. We 

 find, first, the Northfield and Antrim line ; second, the Dunbarton and 

 Peterborough, — areas separated widely by a subsequent covering of rock, 

 but connected directly with the next by an axial line ; third, the Deerfield 

 and Mason line ; fourth, the Hampstead and Pelham. I will notice each 

 of these in order. 



NortJifield and Antrim Range. The north-east end of this range is 

 in several parts, or isolated areas, in S?,nbornton, Tilton, and North- 

 field. West of Merrimack river, the principal exposure upon this first 

 line is met with, occupying a large part of Salisbury, Webster, and 

 Warner, and small portions of Boscawen, Sutton, Hopkinton, and Hen- 

 niker. Next comes an area occupying the Contoocook valley in Hills- 

 borough, Deering, and Antrim. Outliers on a parallel north-east line are 

 in Francestown and Deering, having the Greenfield-Weare-Hopkinton 

 porphyritic ridge for its eastern boundary. On the west side are the two 

 outliers upon the porphyritic gneiss in Bradford, Warner, and Henniker. 

 This line of outcrops is forty-one miles in length, and twelve in its great- 



