322 



Sketch of the Geology of 



in direction with the strata ; but it cuts through their 

 planes nearly at right angles, so that it has a southeast- 

 erly dip of about 30^. This dyke is seen at the surface 

 in one or two places on the Cape southwest of the light 

 houses; and it reappears on Richmond's Island, which 

 is about a mile from the mainland. Another dyke crosses 

 that island parallel to the first. . 



Where the dyke above described appears a little north- 

 west of the light houses, its direction points nearly to 

 Jewell's Island, ten or twelve miles distant. On. visiting 

 that island, I found a dylce at its southwestern extremity, 

 three or four feet wide, coinciding in direction and dip 

 with the strata, and extending across the island. This 

 may be the dyke that appears at the light houses on 

 Cape Elizabeth ; as is represented by the red dotted hne 

 on the map. I am told, also, that a dyke appears on 

 another island eight or ten miles distant, in a northeast 

 direction ; and not improbably, all three may be the 

 same continuous vein. On Jewell's Island, 1 noticed m 

 one place, that the dyke had shifted places, nearly its 

 whole width, without producing any apparent disruption 

 of the adjoining strata. I shall endeavor to explain this 

 anomaly farther on, when I shall describe another similar 

 case. Two other dykes, one of them of greater width, 

 appear on Jewell's Island, having the same general dip 

 and direction as the strata. One also exists near the 

 Portland light house, as may be seen on the map. 



In the mica slate south of Portland, not far from a 

 mile, where a small stream has laid it bare, I noticed a 

 distinct vein of greenstone, not more than two feet wide, 

 whose dip and direction correspond to those of the slate. 

 A little east of the northern extremity of the toll 

 bridge, leading from the north nart of Portland into the 



