312 Sketch of the Geology of 



Pyritiferous Mica Slate. 



F 



Although the plumbaginous mica slate is often pyritif- 

 erousj as are other varieties of the slate around Portland, 

 yet in one or two places on JewelFs Island, in particular, 

 we meet with strata of mica slate so loaded with pyrites, 

 and so affected by its decomposition, as to arrest the 

 attention of the most careless observer. And where this 

 mineral exists in sufficient quantity to render it probable 

 that the rock might be employed for economical purposes, 

 I have denominated it pyritiferous mica slate. I have 

 marked it on the map nowhere but on Jewell's Island ; 

 where it forms at least three distinct beds, several rods 

 wide. On the northwest side of the island, this rock is 

 laid bare for a great extent ; and as the layers are nearly 

 perpendicular to the horizon, it forms a wall from fifteen 

 to thirty feet high, easily accessible, should it be wanted 

 for manufacture. The surface of this rock is disintegrated, 

 often for a considerable depth, by the decomposition of 

 the pyrites, and the formation of sulphate of iron, oxide 

 of hron, and sulphate of alumina and potassa. The cop- 

 peras is perceptible to the taste, almost every where, and 

 the iron gives a rusty aspect to the cliff, as seen from the 

 ocean. The sulphate of alumina and potassa appears in 

 a white efflorescence, where the projecting cliff protects 

 it from the weather. I suspect, however, without having 

 applied any chemical tests, that a part of this efflores- 

 cence is not alum : perhaps it may be sulphate of alumina, 

 which would need the addition of potassa to convert it 



into alum. 



The sulphate of iron, from which the alum and cop- 

 peras In this rock originate, is dissemmated in small 

 grains or crystals through the mass, and rarely forms 



