342 Morris Morris.- 



water, were abundant in the first series (a), and almost absent from 

 seiies b. Steam vesicles are often prevalent in the first series; they 

 are entirely absent from the second. 



Unlike most of the plagioclase felspars, orthoclase, and also' 

 quartz, cannot be produced artificially from simple fusion, except 

 with the aid of water or other mineralisers. The water functions 

 chemically as a catalyser. and physically as a flux, reducing the 

 viscosity of the magma, and lowering the freezing points of its 

 constituent minerals. It is well known that a simple fusion of the 

 constituents of orthoclase becomes extremely viscous in the neigh- 

 bourhood of its melting point. Harker (19) remarks, " It is easy 

 to believe that the presence of a small amount of water may so 

 reduce such viscosity as to enable the alkali felspar to crystallise 

 freely. ' " 



We may, therefore, conclude that the presence of water in the 

 first magma (a), enabled orthoclase to crystallise freely, while its 

 absence from the second magma (b) restricted the process. Since 

 the mineralisers must be regarded as an integral part of the rock 

 magma, the reason for separating the first two series is obvious. 



Evidence of Fo/infio^t. — In the quarry of Allot. 22c on the west 

 limb of the system, series b acquires a very sight banded character, 

 which is only discernible on a polished surface. This feature, in 

 an accentuated form, is quite common in the lower series (a). 



Specimens of series a from Allot. 23* show two sets of banding 

 intersecting at a very small angle. In thin sections, the bands are- 

 seen to be occupied by the ordinary ground mass, and by abundant 

 grains of a sericitic mineral, which is clearly secondary. The 

 phenocrysts, and the few microlites which occur, are arranged with 

 their long axes approximately parallel to the banding. The 

 phenocrysts show scarcely any sign of strain shadows; therefore, 

 the banding was not produced by earth movements after consolida- 

 tion. It must be explained, then, either as the bedding planes of a 

 tuff or as foliation due to movement during consolidation. The 

 parallel arrangement of the microlites and phenocrysts with refer- 

 ence to the banding, and the presence of two sets of bands, offer 

 support to the latter hypothesis, and point to the possibility of these 

 rocks having been involved to some extent in the last stage of the- 

 folding movements. This conclusion serves to con-oborate the 

 petrographic evidence in favour of the Lower Devonian age of th& 

 rocks of this system. 



