272 MR. E. H. RASTALL ON THE [May I906, 



VI. Summary and Conclusion. 



From the facts put forward in this paper, it may be concluded 

 that the Butterrnere-Ennerdale intrusion is an excellent example of 

 an acid magma which has crystallized under the peculiar set of 

 conditions that give rise to a very perfect development of grano- 

 phyric structure. These conditions are probably, to a certain 

 extent, intermediate between those of the plutonic and the true 

 hypabyssal rocks. 



Besides the normal acid rock which composes the bulk of the 

 intrusions, there are some marginal patches of a more basic 

 character, showing obvious genetic relationship, and slightly earlier 

 in point of time than the intrusion of the acid rock. These basic 

 forerunners afford evidence of partial differentiation of the magma 

 before intrusion, an example of Prof. Brogger's deep-magmatic 

 differentiation. There is also some slight indication of lacco- 

 litic differentiation. 



Considered as a whole, the petrographical character of the magma 

 shows closer affinity to the toualite-group than to the true granites, 

 although it is somewhat more acid than the majority of the tona- 

 lites. The more basic types include dolerites, quartz-dolerites, and 

 a rock-type intermediate between quartz-dolerites and gran ophy res, 

 for which no satisfactory name seems to exist. There is also a 

 development of peculiar rock-types, as the result of the re-mixing 

 of previously-differentiated partial magmas of an acid and a basic 

 character respectively. 



A study of the distribution of different types of gran ophy ric 

 structure shows a certain regularity of arrangement, and an attempt 

 is made to reconcile these with known physical laws, especially 

 with reference to eutectics ; and it is concluded that the structure is 

 the result of crystallization under conditions intermediate between 

 those which produce typical plutonic and hypabyssal rocks. 



It was originally my intention to deal also with the study of 

 the very interesting metamorphism brought about in the Skiddaw 

 Slates by these intrusions, but pressure of teaching work during the 

 last two years has prevented me from carrying out this intention. 

 I have collected a large amount of material, and hope soon to be 

 able to continue the investigation. 



In conclusion, I have much pleasure in expressing my hearty 

 thanks to Dr. Marr for much kind help and encouragement, without 

 which the work would have been impossible, and also to Mr. Alfred 

 Harker for much help in the petrological part of the work. My 

 best thanks are due to Mr. W. G. Fearnsides, M.A., E.G.S., for his 

 kindness in taking the photographs for illustrations. I am also 

 greatly indebted to Mr. J. P. Millington, Mr. Bernard Smith, and 

 Mr: B. Schon for much pleasant companionship and help in the 

 field. 



