part 2 J THE XII A P SIINOB 1NTIHSIONS. 141 



lamprophyres cut the felsites, which are there very abundant 

 forming thick sills, and this (together with their position) may 

 indicate their intrusion at an earlv stag-e. Further work in 

 this district is necessary, in order that one may form a correct 

 conclusion. 



The lamprophyres of the west represent two stages of intrusion. 

 These agree with the order so commonly found in the concluding 

 phase of activity — increasing basicity. Both co-existed with 

 felsite ; but, while the enclosures of the earlier lamprophyres 

 contain medium to basic oligoclase, the felspar in the spessartite 

 enclosure is usually monoclinic. Whether this is fundamental 

 or a mere coincidence cannot be stated definitely from the few 

 sections examined ; but it is interesting to observe that they 

 follow the lines which one would expect, on the assumption that 

 they represent complementary deriyatives from the same magma — 

 an increasing divergence in the later members towards the basic 

 and acid ends. 



The most acid, intrusions are the quartz-orthoclase yeins travers- 

 ing the granite, which are comparable with the acid enclosures just 

 described. It is probable that the spessartites and the veins are the 

 extreme complementary products of the magma. 



YTI. Sltmmakt. 



Differentiation under the influence of gravity produced an upper 

 acid and a lower more basic magma. 



Cooling under plutonic conditions caused the separation of acid 

 phenocrysts which, owing to their density, sank to lower and more 

 basic levels, resulting in a concentration at a lower horizon. 



At the commencement of igneous activity the upper layers ot 

 the acid magma were removed by more or less prolonged intrusion 

 or extrusion, the lower and consequently more basic portion of this 

 magma finally consolidating in the channel of flow as the Shap 

 granite, rich in felspars and involving portions of the underlying 

 magma as basic patches. 



The sinking of acid phenocrysts into the underlying magma 

 produced a series of rocks characterized by the presence of acid 

 xenocrysts in a ground-mass of varying chemical composition, the 

 normal increase in basicity due to gravity being offset by the xeno- 

 crysts in diminishing amount. The resultant hybrid series includes 

 rocks ranging from biotite-porphyrites to pilitic lamprophyres, all 

 more or less modified by the addition of xenocrysts from a more 

 acid but allied magma. 



The mixed magma gave rise to the first important group which, 

 with the accompanying acid group, forms the earlier set of intru- 

 sions. These are local in distribution, being restricted to the 

 immediate neighbourhood of the granite, and are distinguished by 

 the presence of large orthoclase-felspars of the granitic type. 



Further differentiation on a regional scale produced after a con- 

 siderable interval a later set of intrusions, ranging from acid 



