A CONTRIBUTION TO THE GEOLOGY OF SOUTHERN MAINE 523 



of which the major ones have a N. 85 E. direction. Dikes 

 follow some of the joints. 



The oldest rocks of the region are mica schists, which are 

 soft and easily eroded. Intruded into these are granitic rocks 

 of various types, which are hard. The metamorphism which 

 occasioned the schistosity followed the intrusion of the igneous 

 rocks, and the present adjustment of streams is in part to the di- 

 rection of the strike of the bands of soft rock, in part to the direc- 

 tion of the fissility. There has probably been some folding also. 



Faulting on a small scale is frequent. Plate XXXI , Fig. 1 

 shows a small example of "Graben" on Negro Island. There 

 are many such, the faults being usually of the gravity type. 

 "Whether there has been faulting on a larger scale, it is impossible 

 to say; but the fact that the eastern shore of the promon- 

 tories is invariably steep, while the western has a gentle slope 

 is suggestive. 



The drainage is well adjusted, and the only notable pre-glacial 

 interruptions to the erosion cycle have been movements of 

 elevation and depression. Evidence of elevation is to be seen 

 in wave- cut cliffs and gorges, but these are at no great height 

 above the present level of the sea. If any such great subsidence 

 took place as has been described by Shaler in Mount Desert 1 it 

 was of too rapid a nature for shore features to be developed. 

 A single marine beach is to be seen on the north side of 

 a hill east of Boothbay Harbor at an altitude of about 100 feet. 



The diabase dikes produce notable topographic features. 

 The dikes are harder than the surrounding schists, and in the 

 interior of the country away from the sea, they stand up as 

 conspicuous ; ridges. The dikes have a columnar parting 

 which is either horizontal or slightly inclined. When within 

 reach of the waves this columnar structure affords oppor- 

 tunity for wave action, and the dike thus attacked is worn 

 away more rapidly than the surrounding rock, thus producing 

 a chasm. These two contrasting topographic effects are illus- 

 trated in Plate XXXI, Fig. 2, and Plate XXXII, Figs. 1 and 2. 



The altitude of the highest hills is of moderate uniformity; 

 it varies from 200 to 290 feet above sea. This level apparently 



1 Annual Rept., U. S. G. S..VIIT, Pt. II. 



