454 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



tion, however, was made by the author between minerals occurring 

 in veins, and those forming the constituents of rock-masses ; so that 

 this paper, and others published subsequently (Trans, of Geol. Soc. 

 vol. hi. p. 455, and of Roy. Soc. of Edinburgh, vol. x. pt. 2. p. 407, 

 Ed. New Phil. Journ. April 1845, p. 386, Phil. Mag. vol. xxxi. Aug. 

 1847, p. 101, and vol. xxxiii. Jan. 1849, p. 489), though of the 

 highest interest optically and physically, can scarcely be considered 

 applications of the subject to pure geology. 



In the * Philosophical Transactions of the Royal Society' (1822, 

 p. 367), Sir H. Davy described the experiments which he made to 

 determine the nature of the fluid contained in the larger cavities of 

 rock-crystal. He found that it was nearly pure water, and that the 

 bubble along with it was either azote or a vacuum. In some cavities 

 he found a kind of mineral oil or naphtha. 



In his ' Note sur les emanations volcaniques et metalliferes ' 

 (Bulletin de la Societe Geologique de France, 2e serie, t. iv. p. 1249), 

 M. Elie de Beaumont alludes to the presence of cavities, with fluid, 

 in quartz, as an argument in support of his views respecting 

 the origin of granite. Similar remarks are also made by Bischoff 

 (Chemical and Physical Geology, Cavendish Society's Translation, 

 vol. ii. pp. 16 and 477) and by Scheerer (Discussion sur la nature 

 plutonique du granite, Bull. Soc. Geol. de France, 2e ser. t. iv. 

 p. 468) ; but they all treat the subject as if these cavities were com- 

 paratively rare. Such as are large enough to be distinctly visible to 

 the naked eye are, indeed, rare ; but when a high magnifying power 

 is employed, it is easy to see that the proportion of many millions 

 to a cubic inch is very common in some minerals. 



Such, so far as I am aware, was the state of this subject when my 

 attention was directed to it by examining the excellent collection of 

 "fluid-cavities" in the possession of Mr. Alexander Bryson of Edin- 

 burgh, who told me he had found some in the granite of Aberdeen. 

 I immediately perceived that the subject could not fail to lead to 

 valuable results when applied to geological inquiries, and soon 

 proved that my supposition was well founded. In my paper on the 

 microscopical structure of mica-schist, read before the British Asso- 

 ciation at Cheltenham, I argued that "the vast number of fluid- 

 cavities containing water indicates that the metamorphic changes 

 have been due to an aqueous process and an elevated temperature, 

 and not to heat alone and a simple partial fusion." (Report for 

 1856, Trans. Sect. p. 78.) 



I, Structure of Artificial Crystals. 



§ 1. Crystals formed from solution in water. 



a. Mode of preparation and examination ; general and special 



characters. 



Great and small are relative terms ; and therefore it will be well to 

 adopt a scale, so that the actual size of the cavities described by such 

 terms may be known. Since a very common size is about y^^th 

 of an inch in diameter, I shall adopt the following scale, and call 



