British Association. 125 



though soda is not usually considered isoniorphous with the other aU 

 kalies ; and that, though the different varieties of alum assume the 

 same form, they are not by all chemists considered to contain the same 

 amount of water of crystallization. — Professor Johnston concluded 

 the discussion by suggesting, that the difficulties which had arisen 

 might be easily surmounted, simply by gentlemen agreeing upon a 

 definition of what did or did not constitute a true alum. 



Section of Geology and Geography, Sept. 13. 



A paper by Mr. Henwood was read, *0n some of the Phae- 

 nomena of Mineral Veins in Cornwall.* This gentleman has for 

 several years examined these phaenomena in that important mining 

 district, often at great personal risk, and has from time to time 

 communicated to the geological world many extraordinary facts 

 which he had determined. He brought before the Section his 

 present statement, in the shape merely of a question for solution, 

 as important to the Cornish mining interest. In mines veins are 

 often heaved out of their regular course, and are traversed by cross 

 courses, causing further irregularity. It was a desideratum of the 

 first importance, that a law should be determined by which a heaved 

 vein could again be discovered ; and the opinion of geologists vvas 

 requested on this point. In Cornwall, it is also the opinion among 

 miners, that veins are of contemporaneous formation with tie rocks 

 containing them, which opinion is opposed to that of geologists in 

 general, who consider veins, in most cases, as caused by disruption 

 from mechanical force. Mr. Henwood exhibited a diagram of heaves 

 in the mines of Dolcoath and Huel Prudence, which he submitted to 

 the Section for solution by means of mechanical movement. He him- 

 self was inclined to consider the veins in these mines as of contempo- 

 raneous origin with the rocks in which they are found. The heaves 

 he hud ascertained to be in one and the same direction ^ and he con- 

 ceived that, had they been caused by a mechanical force, they would 

 be found in different directions. 



Mr. Hopkins believed there was no difficulty in solving Mr. Hen- 

 wood's question, by reference to the operations of mechanical force, 

 but several data were necessary at the outset. He was fully aware of 

 the difficulty of miners recovering lost veins, and that it was highly 

 desirable to have rules laid down for doing so. He entered into a dis- 

 quisition upon the formation of veins in general, and first stated the 

 theories that had been proposed to account for them. One was, that 

 of the Cornish miners, as above stated. Another, that fissures in the 

 rocks had taken place after their consolidation, which fissures had be- 

 come filled subsequently with mineral matter: these fissures being 

 either open cracks, or simply discontinuities of the strata. Mr. Hop- 

 kins referred to his paper in the Transactions of the Cambridge Philo- 

 sophical Society, where tliesubject vvas treated mathematically.* He 

 considered the idea of the contemporaneous origin of veins as having 

 no claim to the name of a theory, from it assigning no physical cause 

 — it does not even call in the aid of electric agency. Suppose even 



♦ Mr. Hopkins has subsequently given a view of his researches in Lond. 

 and Edinb. Phil. Mag., vol. viii. p. 227- 



