DURING THE TERTIARY PERIOD IN THE RRIT1SH ISLES. 145 



surface, cannot be decided from the evidence which I have as yet been able to gather. 

 All I can affirm at present is — (1) that in no single instance have I met with a trace of 

 any acid lava that reached the surface save that of the Scuir of Eigg, which belongs to a 

 period long subsequent to the formation of the basalt-plateaux ; and (2) that where the 

 relative ages of the rocks can be fixed, the acid protrusions are almost invariably the 

 youngest. Indeed, the only exceptions to this rule are the latest basalt-dykes and 

 possibly a few basic injections along the margins of the larger gabbro areas. Hence, 

 while I frankly admit that the large and varied series of acid rocks, which no doubt 

 represents a wide interval of time, may in part belong to comparatively early epochs in 

 the protracted volcanic period, the actual available evidence places the emission of these 

 rocks as a whole towards the end of the volcanic history. This evidence I shall bring 

 forward in full detail, since it necessitates an abandonment of what up to the present 

 has been the general belief in regard to the relative ages of the rocks. 



§ 1. Petrography. 



The classification of the rocks which best harmonises the field-evidence and the 

 detailed study of their mineralogical composition, is one which arranges these volcanic 

 protrusions into two series. In the first, the orthoclase is sanidine, and the rocks range 

 from the most vitreous pitch stone through perlitic and spherulitic varieties to quartz- 

 trachyte. In the second series, which embraces by far the largest proportion of the whole, 

 the orthoclase is always turbid, and in this respect as well as in many others the rocks 

 remind us rather of ancient eruptive masses than of those which have appeared in 

 Tertiary time. They range from flinty felsitic varieties, which are obviously devitrified 

 glasses through different textures of quartz-porphyry into granophyre, and finally into 

 granite. As I have been unable to recognise any essential difference of structure and 

 composition between these acid Tertiary rocks and those of far earlier geological time, I 

 give them the names which no petrographer would hesitate to apply to them if they were 

 of Palaeozoic age. It has long appeared to me that these rocks furnish conclusive 

 evidence of the misleading artificiality of any petrographical nomenclature in which 

 relative antiquity is made an essential element of discrimination. 



1 . Pitchstone and Trachyte Series. 



These rocks, though distributed over a tolerably wide area, never occur in the large 

 masses characteristic of the felsitic and granophyric series. They almost always appear 

 as veins, and usually in the vitreous condition, the only exceptions yet known being the 

 bosses of trachyte which rise here and there through the Antrim basalt-plateau. 



Pitchstone. — This rock is found in veins or dykes which rise through different 

 geological formations up to and including the great granophyre bosses of the Inner 

 Hebrides. It also in one solitary example occurs as a lava-stream, or rather a succession of 

 streams, piled over each other in the ancient river-bed of the Scuir of Eigg. It varies in 



