A CONTRIBUTION TO THE GEOLOGY OF SOUTHERN MAINE 553 



much finer grained than the other dike, and is a peculiar rock 

 with no ferro-magnesian minerals in evidence. Phenocrysts 

 and ground-mass are both of plagioclase while a dendritic form 

 of magnetite makes up a large proportion of the rock penetrating 

 both phenocrysts and ground-mass. The feldspar phenocrysts 

 show a kind of twinning unusual in this mineral, the laths cross- 

 ing each other in a manner resembling the twinning of staurolite. 

 Other phenocrysts are H-shaped, the two vertical arms having 

 a similar orientation, the cross-piece being placed at right 

 angles to the others. A few faint outlines suggest augite, but 

 magnetite now makes up their bulk with a dust of a brownish 

 substance which under a high power seems to be a laminated 

 serpentine. It has a cleavage and is probably antigorite. It 

 does not seem to occupy space formerly held by another mineral, 

 but to be redeposited. 



On the western shore of Ocean Point is another small dike. 

 The dike itself is only about ten inches wide, but a chasm four 

 or five feet wide has been eroded along it (see Plate XXXII , 

 Fig. 2). The gully on this dike is about two hundred feet long 

 and has a strike of about N. 65 E. The dike rock is a dia- 

 base, slightly vesicular on one surface. Microscopically the rock 

 is found to be porphyritic, but the phenocrysts are completely 

 altered. There seem to be three types of alteration product, 

 one of which is kaolin and muscovite and is probably de- 

 rived from feldspar; another is a green serpentine, psobably 

 from augite; and the third a gray serpentine with calcite 

 and quartz, probably from olivine. In the ground-mass is 

 much pyrite and a network of plagioclase with needles of a 

 hornblende which is pleochroic in brown and pink, and a little 

 actinolite. The texture is not typically diabasic, some feld- 

 spar occupying the interstices. 



Some rods farther north is another dike identical with the last. 

 It is only three inches wide but forms a chasm. 



STRIKING PETROGRAPHICAL FEATURES ILLUSTRATED ABOVE. 



The magmatic relationship of the trap dikes to the older 

 metam orphic complex is admirably illustrated on the Boothbay 

 quadrangle. The similarities will be apparent after an inspection 



