Portsmouth Basin, Me. and N. H. 147 



First Group. 



Although this group has been divided into three sub- 

 groups, — diabases, diorites, and granite porphyries, no 

 hard and fast line of division can be drawn, since the dia- 

 bases grade into the diorites and the diorites show transi- 

 tion phases which suggest the granite porplryries. 



Diabases. — For the most part the diabase dikes are the 

 oldest of the region. In the batholiths other diabasic 

 dikes are seen which indicate a second period of diabase 

 intrusion as a late phase of igneous activity. This 

 region illustrates therefore the initiation of a period of 

 intrusion by a great development of diabase dikes and 

 the recurrence of similar rocks at the close of the 

 irruptive period. 



Diorites. — As the rocks in the preceding class grow 

 lighter in color they grade into the diorites. The lighter 

 colored members of the group have a banded appearance 

 due to the development of innumerable segregations of 

 quartz and feldspar in subparallel lines that follow the 

 contacts. The development of quartz in these rocks is 

 worthy of note for it may indicate that the magma of 

 which they are offshoots began to approach a quartz 

 diorite in composition. 



Granite Porphyries. — Under this heading have been 

 placed all of the light-colored typically porphyritic dike 

 rocks of which the phenocrysts are invariably quartz 

 and feldspar. Some of these are characterized by 

 resorbed quartz phenocrysts, by pyrrhotite instead of 

 pyrite, and by a groundmass which consists almost 

 entirely of myrmekite. The myrmekite may well have 

 resulted from the escape of volatile components as has 

 been suggested by Sederholm. 13 



Second Group . 



General Statement. — This group consists of two sub- 

 groups of diaschistic dikes, — the one contains the paisan- 

 ites, tinguaites, and camptonites; the other the aplites 

 and the late diabases. The first sub-group is gener- 

 ally associated with rocks that belong to the alka- 

 line clan; the second with those which belong to the sub- 

 alkaline clan. 



13 Bulletin de la Commission Geologique de Finland, No. 48, p. 81, 1916. 



