44 ON THE PRESSURE CAVITIES IN TOPAZ, RERYL, AND DIAMOND. 



ring in groups, or lying singly with their optical axes in every direction, admit of 

 no other explanation than that which is afforded by supposing the surrounding 

 mineral to have been in a state of fusion, and to have either contained the elements 

 of the embedded crystals, or to have surrounded them when previously formed. 

 Although, as I have already stated, no British geologist has seen the import- 

 ance 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. FouRNET,f 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 effusion. In opposition to this argument, M. Elie de Beaumont makes 

 the following observations : — " It is difficult," says he, " to admit that crystals of 

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

 81° Fahr., have crystallised 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." t M. Fournet^ has, we think, removed this difficulty; 

 but without 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 hoAv 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 im- 

 miscible fluids, the one dense and the other volatile, and in another specimen 

 various crystals, of different 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 surfaces 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 abso- 

 lutely certain that an elastic force, emanating from a pressure cavity, could not 

 have comjz^essed 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 Metamorpliisme," 1860, p. 36. 



f Comptes Rendus, &c., torn. li. p. 42, torn. liii. pp. 83, 610; and Fournet, " Geologie 

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



J Comptes Rendus, &c., 15th July 1861, torn liii. p. 83, note. 

 § Greologie Lyonnaise, p. 536. 



