DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 147 



2. Felsite, Quartz-Porphyry, Granophyre, and Granite Series. 



Felsite and Quartz-Porphyry. — Under the general name of felsite, I class an abundant 

 group of rocks, which macroscopically vary in texture from flinty or horny to dull finely 

 granular, and in colour from white through shades of grey, buff, and lilac, to black, 

 generally with porphyritic felspars and blebs of quartz. Where these porphyritic 

 enclosures increase in size and number, the rocks cannot be distinguished from ancient 

 quartz-porphyries, and I have preferred to call them by that name. 



In no single instance have I found any vitreous variety among them, nor any 

 remnant of true glass in their minute structure. But that many, if not all of them, were 

 originally glasses can hardly be doubted. They often exhibit the most beautiful flow- 

 structure, the laminae being distinctly visible to the naked eye as they curve round the 

 porphyritic crystals of earlier consolidation. Sometimes indeed this structure has been 

 so strongly developed as to cause the rock to weather along the planes of flow and to 

 break up into thin slabs. The passage of the original glass into a lithoid condition does 

 not seem to have been accompanied with the development of those well-marked types of 

 microlites so characteristic of the pitchstones. On the contrary, I have never detected 

 any other modification than that confused and indefinite aggregate which is known as 

 felsitic. Occasionally a spherulitic structure may be observed. More frequently the 

 peculiar radially-fibrous aggregation of quartz and felspar presents itself, which is the 

 characteristic structure of the granophyres. There is thus a gradation from ordinary 

 felsites and quartz-porphyries through granophyric varieties into perfect granophyres. 



The felsites and quartz-porphyries present many of the structures of rhyolites, and 

 would in fact be classed by many petrographers under that name. But I think it better 

 to keep the term rhyolite for those acid lavas wherein the felspar occurs as crystals of 

 sanidine, which, together with quartz, are embedded in a microcrystalline felsitic or 

 vitreous ground-mass. 



There is here again an obvious relation between lithological texture and geological 

 position. Where the acid rocks have been injected into narrow chinks and fissures, they 

 are finer in grain than in the centre of large masses, and have generally consolidated as 

 veins or dykes of felsite or quartz-porphyry. Where they have accumulated in larger 

 bosses, as in Mull and Skye, they have taken the form of granophyres or granites. 

 Along the margins of these bosses, where the conditions of cooling and crystallisation 

 more nearly resembled those in the fissures, the rocks are finer in texture, and not 

 unfrequently assume the felsitic or porphyritic aspect, and even show a more or less 

 perfect flow-structure. 



Granophyre. — This term, which is here used in the sense employed by Rosenbusch 

 (but without his limitation of it to pre-Tertiary rocks), embraces undoubtedly the most 

 characteristic and abundant rocks among the acid protrusions of the Inner Hebrides. 

 These vary in texture from a fine felsitic or crystalline-granular quartz-porphyry, 

 in the ground-mass of which porphyritic turbid felspar and quartz (sometimes bi- 



