480 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



§ 6. Minerals and Rocks formed by the combined operation of 

 water and igneous fusion. 



a. Minerals in the blocks ejected from Vesuvius. 



As is well known, in the blocks ejected from Vesuvius during 

 eruption, a large series of minerals occurs, which do not exist in the 

 erupted lava. Many of these are found in the limestone blocks in 

 the Conglomerate of Somma, and, as pointed out by Delesse (Bulletin 

 de la Societe Geologique de France, 1852, t. ix. p. 136), in their 

 number and character they differ so much from erupted lava, that it 

 is little probable that the rock was ever in a state of simple igneous 

 fusion. This conclusion is completely borne out by the microscopical 

 structure of the minerals, for they contain many fluid-cavities, as 

 well as glass- and stone-cavities, indicating that they were formed by 

 the combined action of water and igneous fusion. 



In the calcite associated with light green mica, I have found many 

 very excellent fluid-cavities, as shown by fig. 78. When heated they 

 give off water, and on reducing the spar to powder, water extracts 

 the chlorides of potassium, sodium, and magnesium, and the sulphates 

 of potash, soda, and lime. Hence I think there can be no doubt 

 that the cubic crystals seen in the cavities are chloride of potassium 

 or sodium, and the fluid a concentrated aqueous solution of those 

 salts. I have not been able to ascertain the relative size of the 

 vacuities with great accuracy, but it is nearly ^th of the volume of 

 the fluid. Similar cavities occur in the nepheline of ejected blocks, 

 and they all contain one or more cubic crystals of chloride of potas- 

 sium or sodium, as shown by figs. 79, 80, and 83, no doubt deposited 

 from the fluid on cooling, like those seen in the cavities formed 

 artificially (figs. 18 and 19). Occasionally there are crystals of some 

 other substance, as in fig. 79. I have very carefully determined the 

 relative size of the vacuities, and find that it is about *28 of the fluid ; 

 and it is so uniform as to forbid us from supposing that the vacuities 

 are owing to a loss of fluid. In calculating the temperature in this 

 and the other cases given below, I shall assume that the pressure 

 was not much greater than sufficient to counteract the elastic force 

 of the vapour, so that we may consider v=Y, and make use of 

 equation (8). Judging from the change in the amount of expansion 

 produced by an increase in the amount of salt in the experiments 

 already described, the temperature indicated by relatively large 

 vacuities would be nearly the same when there was more salt than 

 can be retained in solution at the ordinary temperature, as when there 

 was only 25 per cent. ; the small difference being singularly enough 

 almost exactly compensated for by the increase in the bulk of the 

 salt on crystallizing. This is a fortunate circumstance in this in- 

 quiry ; since, when, as in the case of the fluid- cavities in many 

 modern volcanic minerals, the vacuities and included crystals are 

 relatively large, it is unnecessary to take anything into account but 

 the relative size of the vacuities, and substitute their values in 

 equation (8). In this manner I deduce that this nepheline and 





