vol. 51.] granophyre and the grainsgill greisen. 145 



15. Conclusion. 



In this and my former communication I have described, and to 

 some extent discussed, some of the striking variations observable in 

 the igneous rocks of the Carrock Fell district. While that area 

 affords special facilities for such a study, it is probable that varia- 

 tions similar in kind, and perhaps in degree, occur among other well- 

 known British rocks. The tendency to select in the field a ' typical ' 

 specimen, i. e. one similar to those already exhibited in museums or 

 obtained by dealers from some particular quarry, has perhaps caused 

 us to overlook or neglect in many cases the cousiderable variety of 

 types met with in a single body of eruptive rock. 



As regards the cause of the phenomena, enough has been said to 

 show that many different considerations may legitimately enter into 

 speculation on the subject. The most important kinds of variation 

 described in the foregoing pages are : 



(i) The increasing basicity of the gabbro from the centre to the 

 margin of the intrusion, which is explained by a concentration 

 of the basic elements in the cooler portion of the magma 

 during the progress of crystallization. 



(ii) The relatively basic modification of the granophyre at its 

 junction with the gabbro, ascribed mainly to an incorporation 

 of re-fused ultrabasic gabbro into the granophyre-magma. 



(iii) The passage of the Skiddaw granite into a remarkable 

 quartz-mica rock, which is perhaps a result of mechanical 

 pressure operating on the granitic magma during its period 

 of crystallization, but complicated by later metasomatic 

 processes. 



Some minor kinds of variation have also been noticed, and a 

 study of the numerous dykes and veins, belonging to the latest 

 phase of igneous activity at Carrock Fell, would doubtless bring out 

 further points of interest. 1 



Nothing has been said of possible differentiation in igneous 

 magmas prior to their intrusion, this being a matter of somewhat 

 remote inference rather than a deduction from observation. It 

 may be remarked, however, that all the results arrived at are con- 

 sistent with the hypothesis that the gabbro and the granophyre 

 Avere formed by magmas which were themselves partial magmas 

 derived by differentiation from a common source. Such an idea 

 is indeed suggested, not only by the close association of the two 

 rocks, but also by their chemical composition. The similarity 

 underlying their differences may be partly exhibited by a diagram 

 analogous to those used by Iddings and others, in which the 

 molecular proportions of silica and of the several bases are taken as 

 abscissae and ordinates respectively. I have preferred, however, to 

 plot the atomic proportions of the corresponding elements, instead 

 of the molecular proportions of the oxides ; by this plan the several 

 1 See Geol. Mag. 1894, pp. 551-553. 



