ASSOCIATED METAMOBPHIC BOCKS OP THE LAKE-DISTBICT. 581 



The least temperature at which this granite could have been con- 

 solidated, ifp=0, is 228° 0. (502° F.). I have already stated my 

 belief that geological evidence is strongly against its having been 

 consolidated at a much greater depth than 14,000 feet, at which 

 depth, according to Pox's mean, a temperature of 168° C. (335° F<) 

 would be, under ordinary circumstances, attained. 



It is somewhat striking that this discordance with other similar 

 microscopic results should be in the case of a granite which is easily 

 distinguished from all other granites by the extraordinary develop- 

 ment of large felspar crystals throughout its mass. 



One of the three following suppositions must be true : — 1. The 

 microscopic measurements may be wrong. 2. The application of 

 measurements, even if approximately correct, may be upon a wrong 

 principle, and no results may be trustworthy. 3. The circumstances 

 under which the Shap Granite was formed may have been in great 

 measure peculiar to itself. 



"With regard to the first supposition, it may be answered that 

 measurements taken from very different parts of the granitic mass 

 give nearly the same mean result. In reply to the second it may be 

 urged that it is difficult to believe that the many striking results 

 brought forward by Mr. Sorby are merely coincidences. On the 

 whole it seems most likely that the third supposition is the true 

 one ; and we must proceed to inquire into this possible peculiarity of 

 circumstances. 



In the first place we may infer from the coarsely crystalline nature 

 of the rock, and the development of unusually large and numerous 

 crystals of felspar, that the cooling or consolidation proceeded very 

 slowly, the felspar separating first in a crystalline form, and, by so 

 doing, probably setting free sufficient heat to enable the quartz to 

 retain its soft condition for a time*. Now quartz, in solidifying, 

 passes through a viscous state, while felspar solidifies more rapidly 

 on account of its tendency to crystallize, and passes at once from a 

 fluid to a solid condition f. Hence, may we not suppose that the 

 fluid-cavities occurring in the quartz of this granite were enclosed 

 at a much greater depth than 14,000 feet a depth perhaps equal to 

 that at which the Eskdale Granite was consolidated, and that, while 

 the quartz remained in this viscous condition, — containing fluid- 

 cavities, the granitic magma ate its way up among the overlying 

 strata, aided by the great pressure from below, and finally became 

 solidified at no greater depth than 14,000 feet ? 



If the above supposition have any truth in it, we may further 

 believe that the Shap Granite represents a part of the granitic magma 

 underlying large areas of the district at the close of the Upper 

 Silurian, at depths varying from 22,000 to 30,000 feet, which was 

 enabled to eat its way nearest the surface at this point (Shap) ; 

 whilst in other cases solidification took place at a greater depth, and 

 granite of a different character resulted. If this bo so, we may 



* See Durocher " Sur l'Origine cles Eoches Granitiques," Comptes Eeadns, 

 vol. sx p. 1275, 

 t Durocher. 



