174 ORIGIN OF CRYSTALLINE ROCKS. 



that, in the lengthened periods over which the natural operations 

 extended, the heat involved may have been less than would at first 

 have been expected ; and it is more than probable that the pheno- 

 mena are not to be explained by the action of heat alone. All 

 experiments at the earth's surface are necessarily under a pressure 

 which is infinitesimal in comparison Avith that of the superincumbent 

 rock, which has since been denuded from a granitic district, some- 

 times for a thickness of miles, to say nothing of the force of pressure 

 from lateral contraction which is superadded. And while the water 

 within the rock at once escapes in a furnace experiment, the water 

 is inevitably imprisoned in a metamorphosed rock, so that the con- 

 ditions of the experiments are not the same. The presence of water 

 appears to be necessary to the production of such crystalline forms 

 for minerals as are met with in nature ; for in blast-furnace slags, 

 which are run out at a temperature of about 3700 F., only complex 

 feathery skeletons of crystals are commonly formed, with those 

 minute light and dark needles scattered in the glass, which have been 

 named belonites and trichites. And when igneous rocks, such as 

 basalt, are artificially melted, the augite crystallises in flat feathery 

 plates, like those of furnace slags, which are rarely if ever seen in 

 nature ; and the felspar prisms end in complex fan-shaped brushes, 

 so that the structure of the rock is changed by the conditions of 

 liquefaction and consolidation. 1 Similarly when the Leicestershire 

 syenite is fused and slowly cooled, the solid crystals are lost and 

 replaced by feathery skeleton crystals of magnetite, and flat prisms 

 of triclinic felspar ending in fan-shaped brushes. Dr. Sorby suggests 

 that the difference is due to the presence and absence of water. 2 

 The quartz in schists was found by Sorby to abound in fluid cavities, 

 the fluid being water which usually contains chlorides of potash or 

 soda ; which indicate in the schists of Cornwall a temperature of 

 392 F., and in the schists of the southern Highlands a temperature 

 of 221 F. The quartz of granite also frequently abounds with fluid 

 cavities, so numerous as not to be more than y^TT ^ an ^ ncn a part, 

 so that there may be a thousand millions or more in a cubic inch ; 

 and they constitute 5 per cent, of the volume of the quartz. Similar 

 cavities also exist in the felspar and mica, though they are relatively 

 rare. Crystals of sulphates and chlorides occur in these cavities. 

 Gas cavities and stone cavities both occur. Dr. Sorby remarks, "The 

 proof of the operation of water is quite as strong as that of heat ; and, 

 in fact, I must admit that, in the case of coarse-grained highly 

 quartzose granites, there is so very little evidence of igneous fusion, 

 and such overwhelming proof of the action of water, that it is 

 impossible to draw a line between them and those veins where, 

 in all probability, mica, felspar, and quartz have been deposited from 

 solution in water, without there being any definite genuine igneous 

 fusion, like that in the case of furnace slags or erupted lavas." 

 "While, from the fact that schorl melts readily at a bright red heat, 



1 Sorby, Brit. Association Report, 1880, p. 568. 



2 Sorby, Quart. Jour. Geol. Soc., vol. xiv. 



