Pirsson and Washington — Geology of New Hampshire. 507 



which recrystallized the rock they are minerals which would 

 naturally be least affected and would retain their former shape. 

 Had the rock been a sheared and recrystallized igneous one, we 

 should expect them to have shown crystal outlines. The alter- 

 nation in la} 7 ers of different character points in the same direc- 

 tion. We think therefore that this gneiss was originally a very 

 fine shaly arkose consisting of little-altered granitic debris with 

 occasional very thin layers of a more marly clay-like nature. 

 Often the chemical analysis of a rock helps greatly to decide 

 its original character, but it is evident that the composition of 

 a little altered arkose will be practically the same as that of 

 the igneous rock from which it may have been formed, and in 

 this case the solution of the problem, when it is metamor- 

 phosed, must be sought in other directions such as we have 

 indicated above. 



The Sequence of Magmas. 



This has been previously alluded to on page 350 of the fore- 

 going geological part. Now that the petrologic characters of 

 the different types have been described, the reasons for the 

 adoption of the succession of magmas there given can be dis- 

 cussed more fully. It depends upon the following facts, which 

 have been brought out in these pages. 



a. The pulaskose (syenite) passes into a fine-grained lassen- 

 ose (adamellite aplite) marginal facies. 



h. The pulaskose is cut by dikes of liparose (aplite), of camp- 

 tonose (camptonite) and of akerose (spessartite). The relative 

 ages of these dikes were not determined, as no place was found 

 where they cut one another. 



c. The grano-camptonose (essexite) shows contact facies as 

 it approaches the pulaskose (syenite.) 



d. The grano-camptonose (essexite) is cut by dikes of phyro- 

 camptonose (camptonite) and of liparase (aplite). 



e. The liparase (aplite) of d contains fragments of the camp- 

 tonoses and akerose (essexite and spessartite) and of schists. 



f. The breccia mass is like <?, consisting of -a liparase cement 

 full of masses of the same rocks as in e. 



From these facts we imagine then the sequence of events to 

 have been as follows : First came the intrusion of the great 

 body of pulaskose magma into the gneisses and schists. By 

 processes of differentiation there formed a persalic or more 

 quaric border of lassenose of variable width around this. Then 

 followed a period of cooling and solidification. During this 

 time extended processes of differentiation had been going on 

 deep in the magma chamber, giving rise to products on the 

 one hand more salic, on the other more femic. Then came a 

 second period of magma upthrusting which forced the femic 



