36 B. A. Daly — Mechanics of Igneous Intrusion. 



about 410 calories (assuming latent heat at 90 calories — a value 

 estimated by Vogt for the silicates) per gram must be supplied 

 from an outside source. If all the superheat of the basalt 



55 

 were available for melting (not dissolving) gneiss, of mass- 

 unit of gneiss would be melted by mass-unit of the superheated 

 basalt; or about 7*5 mass-units of the basalt would melt a mass- 

 unit of wall-rock. 



Such simple melting would, however, not occur. There 

 are plenty of field and laboratory proofs that molten basalt, 

 even slightly superheated, will dissolve fragments of gneiss and 

 allied rocks. The mutual solution of two contrasted silicate 

 mixtures takes place at a certain temperature which is lower 

 than the melting point of either one. The simple contact of 

 two such materials suffices to cause their mutual solution at 

 that lower temperature.* This fundamental law of physical 

 chemistry has been experimentally demonstrated for silicates 

 by Yogt and by Doelter and his pupils, although the last men- 

 tioned authors have, perhaps, not sufficiently regarded the fact 

 that it takes considerable time for the mutual solution to take 

 place, f 



Petrasch has experimentally shown that, when two parts of 

 limburgite and one part of granite are mixed and heated, thev 

 melt together at 950° C. and the solution remains fluid down 

 to 850° C.J Predazzo granite softens at 1150° C. and the 

 limburgite at 995° C.§ In this case, there is a lowering of 

 200°-300° below the melting-point of granite and 45°-145° C. 

 below that of limburgite. 



It seems highly probable, thus, that gneiss-xenolith and 

 basalt would form a solution or syntectic film which is molten 

 at a temperature at least 50° C. below the fusion-point of basalt 

 at the average depth of ten kilometers or less below the earth's 

 surface. At those depths basalt melts at about 1100° C. ; the 

 syntectic would be molten at or below 1050° C. If the syntec- 

 tic film were continuously removed during the sinking of the 

 block or by the currents inevitably set up during stoping, 



*Cf. O. Lehmann, Wiedemann's Annalen der Physik, vol. xxiv, p. 17, 

 1885. 



f See J. H. L. Vogt, Christiania Videnskabs-Selskabets Skrifter math.- 

 naturv. Klasse, 1904, No. 1, p. 191 ; and Tscherm. Min. u. Petrogr. Mitth., 

 xxiv. p. 473, 1906. 



%K. Petrasch, Neues Jahrb. fur Min., etc., Beil. Bd. xvii, 1903, p. 508. 

 Petrasch mixed the powders of one part of granite (softens at about 1150° C.) 

 with two parts of hornblende andesite (softens probably about 1050° C.) and 

 found the mixture to become molten at 900° C, proving again an important 

 lowering of the melting-point below that of either rock. Basic rock thus 

 acts as a flux for granite (or gneiss) to an extent comparable with that 

 proved by Petrasch and others for lithium chloride, calcium fluoride, ammo- 

 nium chloride, sodium tungstate, etc. 



§C. Doelter, Tscherm. Min. u. Petrogr. Mitth., xx, 1901, p. -210. 



