176 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



substances, mainly kaolin, owing to its remarkable insolubility 

 and to its finely divided physical condition, has given rise to the 

 " flucan " and "prian" so characteristic of many metalliferous 

 veins. 



Sec. 8. — Subterranean ore-concentration hy heat, pressure, and the 

 crystallizing forces. 



The whole course of modern investigation as to the origin 

 of valuable metalliferous deposits goes to show (1) that the 

 metals sought are very widely distributed through the rocks 

 forming the earth's crust, and (2) that only those deposits are 

 worth working which represent notable concentrations of the 

 said metals. We have now to consider the nature and mode of 

 operation of the forces, or forms of force, concerned in this 

 natural subterranean concentration. 



Sir Greo. Grroves' study of the correlation of the physical 

 forces has been one of the most profitable labours of modern 

 times. iSo much, and. so intimately are they related that it is 

 impossible properly to appreciate the effects of one without being 

 continually led to consider others. " Light runs into heat ; heat 

 into electricity, electricity into magnetism, magnetism into 

 mechanical force, mechanical force into light and heat ; the pro- 

 teus changes, but he is ever the same."* I will, however, 

 endeavour to discuss their actions separately in connexion with 

 rock-change and the formation of ore-deposits, as far as may be 

 possible or convenient. 



1. — Action of heat and pressure. The simple action of heat 

 on rocks is observable when a lava-stream flows over their surface. 

 Clays are locally converted into porcellanite or ferruginous 

 jasper, — as, for instance, at Portrush, where certain beds of 

 lias-clay have been converted into a hard brittle splintery rock^ 

 when in contact with a large body of trap-rock. f Equally 

 marked changes have frequently been noticed in connexion with 

 beds of sandstone. 



But heat alone, without accompanying and considerable 

 pressure, can only act on rocks at the earth's surface, and with 

 little intensity even then, except locally. But the rocks and ore- 



* Tyndall, quoted by Chas. Fox, Address Miners Assoc, 1862. 

 t Jukes and Geikie, Manual of Geology, 1872, p. 140. 



