258 



matite. The batholiths and granitic dikes penetrated the 

 sedimentaries, and were, in turn, penetrated by the later 

 pegmatites and quartz veins, forming together an exceed- 

 ingly complicated order of geological events. 



One of the most attractive fields for investigation in 

 relation to this subject is the pegmatization of th-e Pre-Cam- 

 brian sedimentaries, which isi a special feature of their occur- 

 rence. In addition to the injection of thick dikes and veins 

 of pegmatite, possessing a very coarse crystalline texture, the 

 pegmatitic action has penetrated the schistose rocks over 

 wide areas. In the slates the cleavage seems to have pre- 

 sented the planes of least resistance to the mineral solutions, 

 with the result that the latter has, in many instances, com- 

 pletely penetrated the older slates, depositing, in parallel 

 folia, strings, and lenticles, crystalline aggregates of felspar 

 and quartz, giving the slates a granular or gneissic appear- 

 ance. These lines of intrusive deposition may be almost 

 microscopic in their fineness, or they may swell into lenticu- 

 lar aggregates of large size, causing the slaty laminae to curve 

 around them. As a rule, the pegmatitic material follows the 

 cleavage planes, but at times it breaks across the cleavage 

 and produces a tangential deposition. The gradual passage 

 of these impregnated slates into a highly developed and char- 

 acteristic augen-gneiss can be followed. At Aldgate and dis- 

 trict we have examples of the former type, and at Barossa 

 we have examples of the latter. No evidence could be 

 clearer that gneiss, in some of its forms at least, can be de- 

 veloped under the conditions just described. 



Van TTise, in his great work on metamorphism,"'^ lucidly 

 discusses the origin and phases of pegmatization. He con- 

 cludes that pegmatites are formed in the latter stages of ig- 

 neous intrusions, when the liquid rock becomes increasingly 

 aqueous, and gradually passes into a hot-water solution. He 

 states : — ''From the water solutions true cementation takes 

 place : from the rock solutions, true injection. Pegmatiza- 

 tion comprises these and the intermediate processes. It is 

 not to be expected that under great pressure and at high tem- 

 peratures there is any sharp line of demarcation between the 

 processes of aqueous cementation and igneous injection. 

 At the surface it is usually easy to sharply 

 separate aqueous from igneous action, but deeper 

 within the earth even the strongest rocks are latently 

 plastic. At great pressure heated waters must have 

 power to absorb a quantity of material far beyond that at the 



* A Treatise on Metamorphism, U.S. Geo. Sur. Monog. xlvii., 

 p. 720. 



