180 On the Pressure Cavities in Topaz, Beryl, and Diamond. 



Although, as I have already stated, no British geologist has 

 seen the importance of the preceding facts, and their direct 

 bearing on geological theories, yet they have been recently 

 referred to*, and their value fully appreciated, by French 

 geologists. In a discussion with M. Elie de Beaumont on the 

 formation of mineral veins, M. Fournett, the distinguished 

 Professor of Geology at Lyons, has given a full and interesting 

 account of this class of phenomena, and has adduced them to 

 prove that mineral veins are formed by the injection of mineral 

 matters in the state of fusion. In opposition to this argument, 

 M. Elie de Beaumont makes the following observations : — " It 

 is difficult," says he, "to admit that crystals of quartz contain- 

 ing two oily fluids, one of which is volatile at the temperature of 

 81° Fahr., have crystallized in a bath of quartz in fusion. But 

 quartz forms part of the gangues of the greater number of veins, 

 and quartz with fluid-cavities is far from being a rarity "J. 

 M. Fournet § has, we think, removed this difficulty ; but, with- 

 out entering into the question as one of geology, we may safely 

 assert that difficulties attaching to any theory are not arguments 

 against it, especially if there are only two theories, and if equal 

 difficulties attach to them both. We are so utterly unacquainted 

 with the conditions under which the primitive rocks were formed, 

 with the temperatures which prevailed at their formation, and 

 with the pressures to which they must have been subject, that 

 we are not entitled to charge any theory with difficulties which 

 have their origin in our own ignorance, or in the very nature of 

 the subject. We may never understand how the cavities in 

 topaz have such singular and complex forms as those which I 

 have described and delineated, — how these cavities should contain 

 in one specimen two immiscible fluids, the one dense and the 

 other volatile, and in another specimen various crystals of dif- 

 ferent primitive forms and physical properties. We may never 

 understand how a series of these cavities could have arranged 

 themselves in lines now straight and parallel, now curved and 

 concentric, and now radiating from a centre ; or how strata of 

 these cavities could traverse the topaz in all directions with sur- 

 faces of single or double curvature. We may not be able to 

 explain the special difficulty started by M. Elie de Beaumont; 

 and yet it is absolutely certain that an elastic force, emanating 

 from a pressure cavity, could not have compressed the topaz 

 which surrounded it, unless the mineral had been in a soft and 

 plastic state, or in the state of fusion. 



* Daubree, Etudes sur le Met amor phisme, 1860, p. 36. 

 t Comptes Rendus, &c, vol. li. p. 42; vol. liii. pp. 83, 610. And Fournet, 

 Giologie Lyonnaise, Lyons, 1861, pp. 533, 715. 

 X Comptes Rendus, &c, July 15, 1861, vol. liii. p. 83, note. 

 § Gfologie Lyonnaise, p. 536. 



