part 2] THE SHAP MINOR INTRUSIONS. 12J> 



reason of its higher temperature and more basic composition, the 

 underlying magma will possess a higher degree of fluidity. The 

 sinking crystals being in a state of chemical inequilibrium with 

 their new surroundings, reaction between crystal and magma will 

 result in their partial solution. Sinking continues, until either the 

 crystal is entirely dissolved, or a change in physical conditions 

 brings about complete consolidation. ' The larger individuals may 

 descend a considerable distance before being checked ; but many 

 will be completely resorbed, the few ultimately surviving being- 

 small and well rounded by the action of the magma — action 

 increasing in intensity as the difference in composition becomes- 

 more marked. 



Hence on intrusion the upper layers would yield rocks with 

 numerous porphyritic individuals set in a ground-mass probably 

 but slightly more basic than the granite. In a descending 

 sequence the large crystals would show increasing signs of corrosion, 

 the matrix would become enriched in melanocratic constituents- 

 at the expense of the quartz, the contrast between the dwindling 

 extraneous element and the increasingly important ground-mass 

 would become more and more marked. From porphyrites the- 

 rocks would pass to more basic varieties, the extreme being a more- 

 or less basic rock of which the mineralogical constitution would 

 be determined, inter alia, by the original chemical composition 

 of the parent magma and the degree of differentiation reached,, 

 the extraneous element being reduced to a minimum. 



Thus the composition of any particular intrusion would depend 

 upon the portion of the magma from which it was derived, and 

 the result would be the product of two factors opposite in character, 

 but tending to the same end: (a) a gradual increase in basicity 

 due to original magmatic differentiation, retarded by (b) a 

 decreasing acidification, due to the decreasing relative abundance- 

 of phenocrysts from the upper more siliceous layers. These would 

 tend to produce rocks combining features of both acid and basic 

 types, and a series of mixed intrusions would result, consisting 

 essentially of rocks of a more basic magma enclosing xenociysts of 

 an allied acid magma obtained by settlement under intratelluric 

 conditions. 



Compared with the Potter-Fell type these would show a com- 

 bination of extreme characters, whereas the former exhibit the 

 usual features of intermediate rocks ; they would be mixtures of 

 acid xenociysts and more basic magma, whereas the former follow 

 the law of decreasing basicity in phenocryst and ground-mass; they 

 would be hybrids, the former are transitional varieties. A wide 

 range of composition might be expected overlapping the former in 

 both directions, but especially in the basic. Such is actually the case- 

 in the rocks under discussion, and examples occur illustrating the 

 various stages outlined above. The probability of two such 

 strikingly different types of intermediate rock being developed 

 from the same magma might be questioned : it will be sufficient, 

 however, at this stage to point out that they vary, not only in 

 distribution, but also in point of time. 



