BY W. N. BENSON. 149 



epidote and are probably pseudomorphs after plagioclase. The 

 rod-like structures still persist, but are either less well marked, 

 or are emphasised by the development in them of lines of mag- 

 netite, or they are completely hidden by secondary minerals. 

 Other types of variolite are porphyritic; they contain {e.g., 1007) 

 phenocrysts of decomposed augite and plagioclase, in a confused 

 more or less variolitic base, composed of laths and skeletal forms of 

 felspar, together with a little uralite, epidote, and chlorite; a little 

 secondary quartz is scattered about in small irregular patches. 



Associated with the rocks just described, is a very beautiful 

 variolite which consists chiefly of felspar with a little interstitial 

 augite. The characteristic radiating structure is well shown 

 (Plate XX vi., fig 7). This rock forms the outer part of a pillow: 

 it is unfortunately too decomposed to be suitable for chemical 

 analysis, and there are in it abundant veins of pennine and 

 clinozoisite. The felspar seems to be oligoclase, but its exact 

 composition cannot be determined optically. In another rock 

 with a similar groundmass there are numerous phenocrysts of 

 albite(1078). The freshest rock, and the one that has been 

 analysed(1109), has a texture intermediate between the variolitic 

 and ophitic types, with a few phenocrysts. These consist of 

 andesine; the felspar of the groundmass is rather more acid, 

 being a basic oligoclase, with a small extinction-angle, but with 

 a refractive index greater than that of Canada balsam. The 

 augite is partly ophitic, partly in narrow, irregular grains or 

 prismoids. There is a little scattered chlorite, but isotropic, 

 probably colloidal, chlorite occurs, with a small amount of car- 

 bonates, clinozoisite, epidote, and a trace of pyrites. The analysis 

 confirms the optical evidence of the basicity of the felspar de- 

 veloped, and the absence of any ferric minerals. 



There can be little doubt that these rocks are members of the 

 Middle Devonian series, but their greater richness in lime is not 

 at present explicable. 



The Keratophyres. 



The keratophyres are a varied group of rocks which consist 

 almost entirely of acid plagioclase. They are connected in two 

 ways with the dolerite-spilite series. Quartz-dolerites, becoming 



