Portland and its Vicinity. 325 



than they divide any where else. Tlie best examples of 

 these veins, which I observed, occur a mile north of 

 Pride's bridge, on Presumpscut river, in Westbrook, 



In some places I noticed a fact concerning these veins, 

 that has an important bearing upon geological theory. 

 On Jewell's Island, for instance, in a vein not more than 

 three or four feet thick, the rock in the central parts was 

 decidedly more crystalline than near the walls. Now if 

 the vein was originally injected in a melted state, between 

 the layers of the slate, its outer portions, coming in con- 

 tact with the cold rock, would harden much sooner than 

 the central parts ; and consequently, from all that we 

 know of the chemistry of this subject, we should expect 

 the outer portions to be less crystalline. And, in fact, the 

 specimens are little else than a compact mass, with here 

 and there a small crystalline point ; resembling, indeed, 

 the more sohd recent lavas that have cooled with the 

 access of the atmosphere. 



The uniformity in the direction of these trap dykes, 

 renders it probable that they were of synchronous pro- 

 duction, and the result of a common agency. As to the 

 epoch of its operation, we can only say that it was sub- 

 sequent to the formation of the schistose rocks in which 



■H 4 



the dykes are contained. I doubt not but a more exten- 

 .. sive examination of the geology of Maine would bring to 



^- 



^ 



many 



,i- 



Tertiary Strata. 



I have not met with any rocks in place, in the south- 

 western parts of Maine, that belonged to the secondary, 

 or even transition class. But alon^ the shores of Port- 

 land I have found a few boulders, some of them nearly 



