26 J. G. GoodchUd — Aug en- structure and Eruptive Rocks, etc. 



rocks of sedimentary origin into their succeeding plutonic, trappean, 

 and volcanic forms. 



Once these constituents are restored to the rocks undergoing 

 plutonic metamorphism, the regeneration of the felspar ensues 

 almost as a matter of course ; and it is difficult to see what there is 

 to limit its development except the quantity of silicate of alumina 

 already existing within the rock, and the percentage of matter sub- 

 sequently introduced from without. 



It appears that the earlier stages of development of the felspar 

 take the form of isolated granules occurring interstitially amongst 

 the other constituents, and are contemporaneous with the development 

 of granulitic structure. Subsequently, a further development takes 

 place along the planes of structural weakness as the pressure is 

 gradually relieved, and the felspar takes the form of augen. 



Development of felspar does not end with the formation of isolated 

 angen ; for although the typical and the commonest occurrence is that 

 of eyes consisting of single individuals, it is by no means uncommon 

 to find small nests collected at particular centres, which, in this case, 

 as in the other, evidently mark the points where there has been 

 a local relief of pressure during one of the thrusts. From small 

 knots or nests of this kind every gradation can be traced into those 

 remarkable aggregations of rock-forming silicates to which the term 

 pegmatite is generally applied. True pegmatites, as here understood, 

 are generated within the rock, their constituents graduate inter- 

 stitially into the pai'ent mass, and in every case they date from a period 

 in the history of the rock long subsequent to that of its consolidation. 

 In their composition and their general aspect pegmatites may agree 

 with certain " giant granites," and both pegmatite and " giant 

 granite " agree in the fact that they are usually coarser parts of the 

 ])arent mass, that they are related to this in mineral constitution, 

 and that they graduate into it, so that the line of demarcation between 

 the later rock and the earlier is indefinite in both cases. But " giant 

 granites " are contemporaneous with the last stages of consolidation 

 of the rock in which they occur', and are confined exclusively to 

 plutonic rocks; whereas pegmatites ar-e of much later date than their 

 host, and they occur exclusively in rocks that have undergone more 

 or less deformation by plutonic causes at a period anterior to their 

 development. From intrusive veins they differ in the important 

 fact that in many cases it can be shewn that they both originate and 

 terminate as augen do, within the enclosing rock, instead of being 

 off'shoots of larger plutonic masses adjoining, which is necessarily the 

 case with an intrusive vein. 



Pegmatites, like their component augen, appear to be generally 

 formed along pre-existing planes of structural weakness, as it is 

 along such planes that relief of pressure most commonly ensues 

 as a consequence of lateral thrusts during the upward phases of 

 terrestrial undulations. Hence pegmatites shew a marked tendency 

 to run in rudely parallel zones, which conform to the planes of 

 schistosity, or to any other pre-existing planes of structural weakness 

 existiuff at the time. 



