366 



Johns: On Segregation in Igneoiis RocMs. 



assume that there was no marked disturbance of physico-chemical 

 equihbrium at, the moment of intrusion. If the 'magma were 

 under great pressure its temperature would be higher than that 

 corresponding to the orthoclase critical point, and any -consider- 

 able decrease of pressure at the moment of intrusion-would leave 

 the mass in a superheated condition, and equilibrium would be 

 destroyed. The mother liquor would have commenced to attack 

 the orthoclase crystals until , stability was once more restored. 

 There does not appear to be any sign of this, so we may con- 

 clude that the intrusion was not marked by any great decrease of 

 pressure. In the dark patches, however, the porphyritic felspars 

 are corroded and worn, so we have here clear evidence of a local 

 disturbance of equilibrium. The explanation of the corrosion of 

 the large felspars in the dark patches appears to be that after 

 segregation took place they found themselves surrounded by 

 a mother liquor more basic in character than that from which 

 they originally separated, and commenced to enter into solution 

 again. 



There is, however, another feature in the dark patches that 

 has been mentioned but not discussed. Quartz is the last 

 mineral to separate out in the segregation areas, but appears 

 before the later orthoclase in the normal granite. The silica 

 content of the normal area we have seen is higher than that of 

 the segregation ones, but it is not well to rely too much on that, 

 for it would imply that quartz would be more liable to be the 

 last to separate in the normal granite, while the opposite is the 

 case. In a reciprocal solution it is the excess constituent in the 

 unsolidified portion or mother liquor at each stag-e of consolida- 

 that has to be dealt with. In the segregation areas the result of 

 the successive formation of the several minerals is that some 

 silica is the excess constituent at the last, and appears as quartz, 

 but leaving traces of the entectic represented by the occasional 

 occurrence of a micro-pegmatitic intergrowth of quartz and 

 orthoclase. In the normal granite the conditions towards the 

 later stages of the consolidation of the rock are such that the 

 excess silica is thrown out from a mother liquor corresponding 

 in composition to the later orthoclase, which was then left to 

 crystallise out after the formation of the quartz: Owing to 

 limitation of space it is not possible to deal with other interesting 

 features of this rock. The writer, however, hopes that enough 

 has been done to make it probable that no difficulty should be 

 experienced in considering the igneous rocks as reciprocal 

 solutions. The task would be much simplified if all investi- 

 gators were as careful as Messrs. Harker and Marr in their 

 observations. 



Naturalistj 



