Portland and its Vicinity. 309 



The mica slate around Portland is not very well char- 

 acterized in general. Often, it takes so much talc into its 

 composition as to perplex the observer. In other places 

 if is so siliceous, that one can hardly say whether it 

 should not be called quartz rock. Again, and perhaps 

 this is most frequent^ it has that glazed appearance which 

 characterizes the oldest argillaceous slate ; and perhaps it 

 does sometimes actually pass into that rock. There are 

 two varieties of rocks which I have denominated mica 

 slate, in the vicinity of Portland, which, on account of 

 their probable economical value, deserve a distinct notice ; 

 and I have given them distinct colors on the map. 



Plumbaginous Mica Slate. 



Ever since the first settlement of the country, this rock 

 has excited the attention of the inhabitants, on account 

 of its resemblance to the slate that is associated with 

 coal. And, indeed, where the road passes over its sur- 

 face, so that the wheels grind it into fragments, the ap- 

 pearance can hardly be distinguished from that of dism- 

 tegrating slate. But on careful examination, the geolo- 

 gist finds that Its surface exhibits too much of a shining, 

 plumbaginous appearance, so common in the primary 

 slates, to be considered a slate. It does, Indeed, abound 

 in carbonaceous matter : but this seems to be uniformly 

 in the state of plumbago; which, In several places, as at 

 Diamond Cove, on Hog Island, and on Jewell's Island, 

 exists in thin layers. In several other places in the 

 vicinity of Portland, as at Gorham, this mineral is found 

 in a quite pure state; and I confidently expect that it 

 will ere long be found in that vicinity, m large quantity. 

 In short, to give the result of mv observations and reflec- 



r^- 



