PRESSURE CAVITIES IN TOPAZ, BERYL, AND DIAMOND. 48 



others, more recently discovered, have been widely circulated in British and 

 foreign journals; and yet none of our geologists have made the slightest refer- 

 ence to them, either as difficulties to be explained, or arguments to be advanced 

 in support of their own views. 



In 1822 Sir H. Davy, when he was acquainted only with the existence in 

 minerals of water, petroleum, and gas, did not hesitate to regard such facts as 

 " seeming to afford a decisive argument in favour of the igneous theory of crys- 

 talline rocks ;"* and in my paper of 1826, I was driven to the conclusion, " that 

 the cavities containing the two new fluids were formed by highly elastic sub- 

 stances, when the mineral itself has been either in a state of fusion, or rendered 

 soft by heat." At this time I was acquainted only with the two new fluids, and 

 some of their chemical and physical properties ; but when I had studied their 

 arrangement in strata, this opinion acquired additional weight. Had these cavi- 

 ties been arranged in planes parallel to the primitive or secondary faces of the 

 crystal, some argument might be urged in favour of their aqueous formation ; 

 but when it was found that the strata of cavities traversed the crystal in all 

 possible directions, that they were bent also into curves of contrary flexure, and 

 that even individual cavities had a curvilinear shape, it was impossible to resist 

 the conclusion that the cavities were formed, and thus capriciously distributed, 

 when the substance of the crystal was in a soft or plastic state. This conclusion 

 derives additional strength from the fact that the water cavities in crystals depo- 

 sited from an aqueous solution are never thus arranged. 



The discovery of pressure cavities in topaz and diamond may be considered 

 as completing the evidence for the igneous origin of these minerals, and of the 

 rocks which contain them. We know that gas, in a state of compression, exists 

 in minerals. In the pressure cavities we have not only the seat of an elastic 

 force, but its direct action upon the substance of the crystal. Though of equal 

 density throughout, as is proved by the equality of its polarised tints, the crystal 

 has its density increased round the pressure cavity, — the density being a maximum 

 close to the cavity. Such a structure is impossible in crystals formed by aqueous 

 deposition, and hence there is not a single example of a pressure cavity in any of 

 them. They exist, however, in amber and in glass — substances that have once 

 been in a plastic state ; and I have produced them artificially by compressing a 

 solution of gum arabic between two plates of glass, so as to include some bubbles 

 of air. The air in these cavities, being exposed to changes of temperature, com- 

 presses the circumjacent gum, and gives it that variation of density which pro- 

 duces four luminous sectors in polarised light, exactly of the same character as 

 those which are found in topaz and diamond. 



The existence of crystals of different physical properties in the cavities of 

 minerals, and of embedded crystals either shooting through their mass, or occur- 



* Philosophical Transactions, 1822, p. 367- 

 VOL. XXIII. PART I. M 



