Johns: On Segregation in Igneous Rocks. 



365 



not confined to one particular region and often have sharply- 

 defined boundaries, so their resemblance becomes still more 

 striking-. This feature is taken to imply that the mass was 

 partially solidified, otherwise gravitation would have come into 

 play, for their specific gravity is different from the normal mass. 

 These features have been explained by discussing it as a case of 

 reciprocal solutions, and it might be interesting to apply the 

 same theory to a well-known rock like the Shap Granite and see 

 if it applies. It is not proposed to give a list of references, for 

 this was done by Messrs. Harker and Marr''^" in their valuable 

 paper on this rock, from which most of the data used by the 

 present writer was obtained. Briefly, the rock contains large 

 porphyritic pink felspars in a ground mass of early orthoclase, 

 plagioclase, quartz, and later orthoclase. Distributed through 

 the rock are dark patches or segregation areas, more basic in 

 character, but still containing the large pink felspars. There is 

 this important difference between the normal and segregation 

 areas : that in the former the quartz appeared before the later 

 orthoclase, while in the latter the quartz is the last to separate 

 out, though the silica content is lower than in the normal mass. 



Petrologists are by no means agreed as to what it is that 

 determines the order in which the various minerals separate out; 

 the order of basicity has many supporters, while the even more 

 impossible view that the fusion point of the minerals play a 

 prominent part finds adherents. From the variation in the order 

 in which quartz appears in the dark patches and in the normal 

 areas of the Shap Granite, it would appear that neither basicity 

 nor fusion point are determining factors, and it might not be 

 out of place to see if the solution theory will apply. The general 

 and regular distribution of the porphyritic pink felspars through 

 both normal and segregated areas implies that they were formed 

 as a result of the slow cooling of the mass through the critical 

 point of the cooling curve corresponding to the separation of 

 that mineral from the mother, liquor. The conclusion of 

 Messrs. Harker and Marr, that the formation of the large 

 felspars was prior to intrusion, seems to be well founded. 

 There is further evidence that after intrusion cooling took place 

 at a very slow rate, and this, coupled with the viscid condition 

 of the mass, would complete the conditions necessary for segre- 

 gation to take place. It might be pointed out that the distribution 

 of the porphyritic felspars is very similar to that of Tin-antimonide, 

 in the alloy of 80 per cent, tin and 20 per cent, antimony. 



It seems a permissible deduction, from the fact that the large 

 felspars are not rounded or corroded in the normal areas, to 



1905 December i, 



*Q.J.G.S., 1891, p. 266. 



