250 



surface of the earth. Truly liquid rock is highly impregnated 

 with water. It, therefore, is probable that at consideralilc 

 depths we have, on the one hand, material which all would call 

 water solution, and on the other hand material which all would 

 call liquid rock, with no sharp division-line between the two. 

 If this be so, there are all stages of gradation between true 

 ioneous injection and aqueous cementation, and all the vari- 

 ous phases of pegmatization may thus be fully explained."* 



It is an interesting circumstance that \^an Hise and 

 others have observed in the United States a schistose im- 

 pregnation which appears to be precisely similar to that which 

 is exhibited in the Pre-Cambrian slates of SouthAustralia. He 

 says : — ''This phase of pegmatization [the aqueo-igneous] is 

 most extensive and best illustrated by rocks in which there is 

 a gneissic or schistic structure, since cleavage furnishes plane-s 

 of weakness which are readily taken advantage- of by the ig- 

 neous rocks. . . . Parallel to the folia are innumerable 

 cementation-injection bands of lighter colour. These bands 

 vary from those as thin as leaflets, being perhaps but a single 

 row of crystals, to those of considerable width. There may be 

 many such bands within the space of a centimetre, or a single 

 one may be many metres across. Frequently parts of the in- 

 jected material are in dike-like masses of varying size, which 

 cut the schistosity at various angles. At numberless pla-ces 

 the leaf-like bands of pegmatitic-looking material parallel to 

 the schistosity are found to be connected directly with the dike- 

 like masses cutting the schistosity."! 



The final stage of this aqueo-igneous process is when the 

 liquid residuum is distinctively a water solution, and is an agent 

 of simple cementation, penetrating fissures and cavities caused 

 by mechanical strain and porous beds, depositing quartz 

 either diffused or in veins. No better illustration of this class 

 of hydrothermal action could be had than occurs in the inti- 

 mately reticulating veins of quartz, -which penetrate the slates 

 bordering the pegmatized areas of Aldgate. The weathering 

 of the slate has freed the quartz from the matrix, strewing 

 the ground with the scattered fragments, and in bare places 

 giving the resemblance to a light cover of snow. 



As accessory minerals in the peo^matized rock, the most 

 prevalent are ilmenite and tourmaline. Both occur as in- 

 clusions of quartz. The ilmenite is in orains and plates, some- 

 times in considerable quantity. Tourmaline occurs, for the 

 most part, as long acicular crystals of black colour. These 



'^ A Treatise on Metamorphisiii. X'.S. Ooo. S;in-. ^Fonoo;. xlvii.. 

 p. 723. 



I Ihid., p. 725 

 j2 



