i 



Portland and its Vicinity 



321 



inclined to believe this to be the case in respect to the 

 Hallowell granite. In respect to the more easterly parts 

 of Maine^ lioweverj I shall wait for light^ as to its geology, 

 from the expected memoir of Dr. Charles T. Jackson. 



Greenstone. 

 Of that variety of hornblende slate called primary 



greenstone 



rock 



am now going to describe exists exclusively^ so far as I 

 know, in veins or dykes. And these form one of the 

 most interesting features In the geology of the region 

 around Portland. For the most part, they coincide 

 nearly in direction with the strata, and pursue a very 

 straight course for a great distance often. Yet they are 

 real veins; that is, they were evidently intruded among 

 the strata subsequent to their consolidation. They have 

 a structure more or less columnar ; the columns always 

 lying directly across the vein : and since this usually de- 

 scends almost perpendicularly, they lie nearly horizon- 

 tally. In some cases, the dyke on the coast seems to 

 have yielded more readily to the action of the sea than 

 the adjoining rock; so that the weaves have formed a 

 chasm ; as at the north light house on Cape Elizabeth, 

 about four miles from Portlands In most cases, how- 

 ever, as on the northeast part of Jewell's Island, the 

 dyke remains like a wall, while the surrounding rock has 



disappeared. 



About eighty rods northwest of the most northerly light 

 house near the extremity of Cape Elizabeth, we meet 

 with one of these dykes, from six to eight feet wide, in 

 talcose slate. The slate dips about 60^ northwesterly, 

 and runs northeast and northwest. This vein coincides 



VOL. I. PART III. 42 



