II 



THE LARGE FELSPARS OF 5HAP 

 GRANITE. 



COSMO JOHNS, F.G.S., M.I.Mech.E. 

 Sheffield. 



In a recent discussion* of certain features in Shap granite 

 allusion was made to the large pink crystals of orthoclase 

 that characterise this beautiful rock. It is now proposed to 

 endeavour to trace the life history of these crystals in some 

 detail in the light of the theory of reciprocal solutions. From 

 various observations on slags it is now known that felspar 

 forms very slowly, and that alumina exercises a retarding 

 influence on the crystallization of such masses. Seeing that 

 felspar occurs in such abnormally large crystals in Shap 

 granite it becomes very evident that the conditions under 

 which they formed were abnormal. 



From the fact that the texture of the rock does not get 

 finer towards the margin ; that the porphyritic felspars are well 

 developed there too ; that these felspars are wanting in the 

 small ramifying fissures while in the intrusive mass itself 

 they are orientated as if to suggest a line of flow, it seems- 

 reasonable to conclude that they did not form in situ in contact 

 with cold walls. They were probably ready formed when the 

 mass was intruded, and thus date their formation back to the 

 original magma. From the papery quoted in the former article 

 we learn that "these large felspars enclose crystals of apatite 

 and sphene, besides occasional flakes of mica and prisms of 

 striated plagioclase. More rarely they contain little patches of 

 quartz, or even a well-bounded crystal of this mineral." 



With this data at our command, and treating the original 

 magma as a reciprocal solution, several facts seem to stand out 

 clearly. To begin with, the fused mass was once at a higher 

 temperature ; for the growth of the large felspars only became 

 possible when the temperature fell to the point where the 

 orthoclase forming material would separate out. In the earl}'" 

 stages of cooling, apatite, zircon, some magnetite, and possibly 

 sphene, would separate out for the very good reason that the 

 solubility of these minerals in fused silicates, such as the 

 magma was composed of, is a function of temperature. In the 



* Naturalist, December, 1905, p. 364. 

 t Q.J.G.S. 1898, p. 278. 



1906 January i. 



