XXXIV PEOCEEDINrGS OF THE GEOLOGICAL SOCIETY. 



constitution, and structure of rocks. On questions of this sort our 

 obligations are eminently due to the geologists of France and Ger- 

 many. Cordier and Brongniart, Leonhard, Senft, and Cotta, have 

 founded upon the mineral constitution of rocks systems of classifica- 

 tion and nomenclature, especially for rocks of igneous origin, more 

 definite than those commonly employed in this country. Here, on 

 the other hand, we have gradually advanced to the first place in the 

 history of rock- structure, and especially of that interesting part of 

 it known as slaty cleavage. 



The existence of canities containing fluid has been long since 

 ascertained in many minerals of frequent occurrence, as, for example, 

 fluor-spar and quartz, the former an abundant crystallized mass in 

 the lead-mines of the north of England, the latter very common in 

 the tin- and copper-mines of Cornwall. To Sir David Brewster we 

 owe the first accurate steps in the delicate and difficult research 

 into the nature of the liquids and gases which fill these cavities. 

 The cavities in quartz are sometimes largo enough to allow of the 

 contents being examined chemically. Sir H. Davy found the liquid 

 to be water with saline matter; Mr. Eox, Avater with chloride of 

 sodium ; others have discovered water with various earthy aud me- 

 tallic sulphates and chlorides. 



The cavities are seldom full of the liquid — there is usuallj^ a bubble 

 of air, which, except when the cavity is very small, changes place 

 when the position of the crystal is altered. We may suppose with Mr. 

 Sorby that the cavity was originally filled with the liquid, when the 

 consolidation of the crystal happened, at a temperature more or less 

 elevated above the actual temperature ; that it has since contracted 

 dui'ing the cooling, and now occupies a space which, as compared 

 to the whole cavity in the crystal, is determined by the actual tem- 

 perature as compared to the original temperature of consolidation — 

 nearly in this proportion, not strictly, because pressure alters the 

 bulk of a liquid, and the pressure during the formation of the crystal 

 is unlinown. 



The effect of pressure, however, on the volume of water is known ; 

 supposing a case in which the pressure is known not to have been 

 great, the temperature of consolidation may be found approximately. 

 In the case of igneous and metamorphic rocks, Mr. Sorby finds this 

 temperature unequal in different cases, but rising upwards to the 

 heat of dull redness. 



Assuming in this way some fixed temperature as that of consoli- 

 dation for a given crystal, and measuring the relative bulk of the 

 liquid and the cavity, we can determine the pressure, and conse- 

 quently the depth, under which the rock was formed and solidified ; 

 and this depth in the case of granite is found to be so great as to 

 correspond well to the opinion which has been adopted on other 

 grounds of its deep-seated plutonic origin *. 



Far down in the earth, then, water has been present during the 

 formation of the rock — that is to say, the elements of water ; and 



^^ See Mr. Sorby' s investigation, worked out numerically, in Quart. Journ. 

 Geol. Soc., Nov. 1858. 



