310 Sketch of the Geology of 



tions upon the remarkable rock under consideration, 1 

 reo-ard it as slate, or fine micaceous sand stone, which, 

 by the agency of internal heat and other causes, has 

 passed into the condition of an imperfect sort of mica 

 slate; and that the carbonaceous matter, by the same 

 agencies, has become plumbago. After having examined 

 the remarkable bed of anthracite and plumbago at Worces- 

 ter, Mass., which occurs in mica slate, I hardly dare pre- 

 dict that anthracite will not be found in the very oldest 

 slates. Yet the Portland rock appears to me to be older 

 than that in Worcester; and I strongly incline to the 

 opinion, that no carbonaceous mineral, but plumbago, will 

 be found in the former. 



It is a fact which ought to be noticed, that such is the 

 position of the rocks in the vicinity of Portland, that 

 boring perpendicularly into them, to discover coal or 

 plumbago, must, as every geologist will see, be entirely 

 useless. For the strata stand nearly perpendicular to the 

 horizon ; and it is w^ell known, that neither of these 

 minerals occurs except in interstratified beds ; so that 

 by merely uncovering their elevated edges, they will 

 most assuredly be discovered, if they exist. And since 

 the soil above the plumbaginous slate is usually thin, the 

 expense of such an exploration must be small. Should 

 it be attempted, it ought to be recollected that these 

 minerals will be most likely to occur in those places 

 where the soil is deepest ; that is, in the valleys ; since 



they are more easily abraded than the rock. 



The plumbaginous mica slate abounds in iron pyrites, 

 in small disseminated masses and crystals. This is very 

 liable to decomposition ; so that often the rock appears 

 minutely porous* In some instances a black powder 

 results from this decomposition ; so that perhaps there is 



