364 



Scientific Geology. 



a fact worthy of notice, that this rock in Scotland, where it is traversed 

 by quartz \eins, abounds in titanite ; showing a very great similarity 

 in the causes by which it was produced in distant countries. 



Blue a^id green carbonate of copper are found in several places in 

 Cumberland, R. I., in what I have described as talco-chloritic slate. 

 The same rock abounds in epidote, sometimes finely crystalline, ac- 

 cording to Dr. Webb. And I infer from his description, that the ye- 

 nite associated with magnetic oxide of iron, is contained in the same 

 rock. 



Theoretical Considerations. 



The views that have been presented relative to the origin of mica 

 slate, are applicable, almost without variation, to talcose slate. The 

 arguments proving that water must have been the earliest agent 

 in the production of mica slate, prove the same of talcose slate; 

 as do those evincing the subsequent action of a high degree of heat. 

 To what circumstance the great abundance of magnesia in the talc- 

 ose slate is owing, it may be impossible perhaps ever to determine. 

 But its presence being once admitted, it is easy to conceive how talc- 

 ose, instead of mica slate, might have resulted. 



There is, however, one variety of talcose slate, which occurs half 

 a mile west of the meeting house in Hawley, and affords an evidence 

 not found in our mica slate, of the action of heat sufficient to produce 

 an almost perfect fusion. I refer to the porphyritic variety, which I 

 do not find described in European works. It seems to me that every 

 thing which we know of the chemistry of crystalization, forbids the 

 supposition that the porphyritic structure can ever result from any 

 other than an igneous solution. For in what laboratory have distinct 

 crystals been produced in the midst of a mass essentially uncrystal- 

 ized, except from heat ? But it is well known that the porphyritic 

 structure is not unfrequently met with in rocks whose volcanic origin 

 is certain ; even in the products of existing volcanoes. What, then, 

 but an unreasonable attachment to hypothesis, should lead us to im- 

 pute that to watery solution, which, so far as facts have come to our 

 knowledge, has never resulted but from igneous solution ? 



In the present instance the almost entire absence of stratification 

 and slaty structure in the rock referred to in Hawley, gives additional 

 probability to the idea of its having been once in a state of fusion. 



