520 



Scientific Geology. 



Here terminates my account of the particular rocks of Massachu- 

 setts. A few miscellaneous matters that could not properly be intro- 

 duced in any other place, will be added in conclusion of the Scientific 

 Geology of the State. 



Miscellaneous Items. 



1 Origin of Metallic Veins, Beds, fyc. 

 The metals sometimes occur in the rocks disseminated through their 

 masses : and in this case it is obvious that they must have existed in the 

 materials out of which the rock was produced, and were separated into 

 small masses by chemical affinities when the rock was passing from 

 a fluid to a solid state. The same was probably true in those cases 

 where the metals exist in tuberculous masses in the rocks. When 

 they occur in strings, that is, in small veins so numerous as to give the 

 rock a reticulated appearance, they were probably segregated from the 

 mass of the rock at the period of its formation ; not improbaby filling 

 up the small cracks produced by incipient consolidation. Metallic 

 beds, where the ore is interstratified with the rock, may in many cases 

 have resulted from aqueous deposition, the ores having been subse- 

 quently modified by exposure to heat : as I have more fully suggest- 

 ed in describing talcose slate. But when the metals occur in 

 genuine veins, as they usually do, the theory of their origin is invol- 

 ved in great obscurity. The Wernerian dogma, that all veins were 

 filled by aqueous solution from above, is now exploded ; though in a 

 few instances they may have been formed in this manner. A more 

 recent and very ingenious hypothesis represents the contents of me- 

 tallic veins as having been secreted from the rocks by means of gal- 

 vanic electricity;* and the change that takes place in these contents 

 as the vein passes into different rocks, certainly lends some plausibil- 

 ity to this suggestion : and it would seem probable that the various 

 layers of rocks and minerals that make up the crust of the globe, 

 must form galvanic combinations of great power. A still more re- 

 cent hypothesis! imputes metallic veins to sublimation by the inter- 

 nal heat of the earth, which causes the metallic substances to rise into 

 the fissures that exist in the crust of the globe. This hypothesis, 



* Fox in the Philosophical Transactions for 1830 p. 399. 

 t Neckar in Philosophical Magazine. Sept. 1832. 



