THE ORIGIN OF THE INCLUSIONS IN DIKES 



173 



thin section to consist principally of microperthite and oligoclase 

 feldspars, with chlorite and other alteration products and a few 

 grains of quartz; slate similar to the slate adjoining the sill; and 

 vein quartz evidently derived from the veins which cut the slate. 

 The size of the inclusions ranges from a few inches to 4 feet in 

 length, in various parts of the sill. Thus, the inclusions in the 

 part shown in the left-hand section of Fig. i , where the sides of the 

 sill are chilled, have an average size of 8 inches long by 4 inches 

 wide. Nearer the sea, the margins of the sill do not show chilling, as 





Fig. I. — Two portions of the inclusion-bearing sill at Ogunquit, Maine. In the 

 left-hand section the walls of the sill are chilled and the fragments show fused contacts 

 but not resorption. In the right-hand section — about 100 feet away — the inclusions 

 are being resorbed and their edges are indistinct. The width of the sill in the right- 

 hand section is 3j feet. 



shown in the right-hand section of Fig. i, and here the average 

 length of the fragments is about i foot. In the first area the inclu- 

 sions have subangular to rounded outlines and tightly sealed con- 

 tacts. In thin section the quartz from the aplitic granite can be 

 seen to have migrated a sixteenth of an inch into the labradorite- 

 augite-biotite matrix of the sill, but the feldspars from the coarse 

 syenite have not migrated. In the second area, the inclusions all 

 have corroded margins and the keratophyre may be seen to have 

 worked its way into the coarsely crystallized syenite and granite. 

 Blocks of the pink apHtic granite 3 and 4 feet long respectively 

 and about i^ feet wide, with their longer axes orientated in some 

 cases perpendicular to, and in other cases parallel to, the walls of 

 the sill, where the latter is 3I feet wide, show that they have been 



