STRATIGRAPHICAL GEOLOGY. 



PORPHYRY. 



The term porphyry has long been used to denominate rocks with a 

 homogeneous, compact, or earthy base, through which are disseminated 

 crystalline masses of the same character with the matrix, and formed at 

 the same time with it. The porphyry of the White Mountains seems to 

 be allied to a common orthoclase feldspar porphyry, orthophyre, or petro- 

 silex. In the annual reports it has been termed felsite, and grouped with 

 the Labrador system. The localities of this rock are mostly far away 

 from settlements, so that it has not been easy to examine its relations to 

 other formations. The rock has a tendency to weather into pinnacles or 

 sharp ridges like the Twins, Mts. Liberty, Flume, the Lafayette range, 

 Starr King, etc.; and the decomposition extends so deeply into the ledges, 

 that one may obtain from a hasty visit to a mountain a very inadequate 

 idea of its composition. Reserving all special details of nomenclature 

 and composition to the chapter on lithology, we will now be content to 

 describe briefly the areas where it occurs. 



As porphyry occurs both as an igneous and a stratified rock, it is dififi- 

 cult to know to which class to refer this member. Our first impression 

 suggested the idea of stratification, based upon the horizontal arrange- 

 ment of the masses. Of late some facts have been brought to our notice, 

 seeming to imply protrusion through strata, and lateral expansion by an 

 overflow. This theory would explain the horizontal situation of so much 

 of the rock, while it is a fact that no other kind of igneous rock has been 

 found to assume the position of an overflow in New Hampshire. 



It may be that the porphyry mountains of New Hampshire should have 

 been mentioned in the chapter on Scenographical Geology, as constitut- 

 ing a distinct type of rock sculpture. The peaks are somewhat like the 

 isolated granitic summits, yet a practised eye would never confound the 

 two together. The latter are usually more rounded: the former might 

 easily constitute a pinnacle. The porphyry summits remind one of a 

 triangle, with one slope much steeper than the other. Sometimes two 

 summits are connected together by a surface line, like a catenary curve, 

 as seen from a distance. Other areas, like the Twin mountains and Lafay- 

 ette arc disposed in the form of long, narrow ridges. Mt. Carrigain might 

 easily be confounded with a granitic cone, at a distance, but not so the 



