Io8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1 894, 



the Propylites of the "Western Isles, relate to matters which have 



been, more or less touched upon by Sir A. Geikie in his second 



Presidential Address, and consequently I need not bring them 



prominently forward on the present occasion. The second of these 



papers was mainly written for the purpose of showing that the 



' felstones,' described by the Author in his first paper as constituting 



the oldest series of the Tertiary volcanic rocks of the "Western 



Isles, belong to that variety of the andesites known as propylites. 



These rocks, when found in an unaltered state, present remarkable 



analogies with the andesites of Iceland and the Faroe Islands, 



whilst in the altered condition' in which they usually occur the 



propylites of Scotland resemble those of Eastern Europe and other 



regions. A detailed description ensues, and the Author states that 



these rocks exhibit every gradation in minute structure from holo- 



crystalline forms (diorites) through various ' granophyric ' types 



into true vitreous rocks such as pitchstones ; whilst, by carefully 



following in the field the much-altered rocks to points where they 



retain some of their original characters, the propylites can be 



shown to represent various interesting types of andesite and diorite. 



The chief agent in producing change in these rocks he considers 



to have been solfataric action, and this was shown to have 



accompanied the intrusion into the andesites of masses of igneous 



material, acid in composition, such as granites and felsites. 



Forming a very striking contrast with the older Tertiary 

 andesites (propylites) are the numerous scattered and generally 

 small masses of rock, which belong to a late epoch in the Tertiary 

 volcanic period, and constitute the youngest eruptive rocks of the 

 British Isles. One interesting example is afforded by the Scuir of 

 Eigg. These rocks have the mineralogical constitution of the 

 augite-andesites, but differ from the older series in the relative 

 proportion of crystalline and glassy constituents. Thus, noncrys- 

 talline aggregates of basic composition are found passing, as the 

 quantity of acid glass increases, through various phases until they 

 finally assume the vitrophyric form of pitchstone-porphyries. The 

 rocks of the Tertiary dykes in the South of Scotland and the North 

 of England were shown to agree with these later Tertiary andesites, 

 both in their mineralogical constitution and in the peculiar phases 

 which they exhibit. 



In his paper on Composite Dykes in Arran, read at the close of 

 last session, Prof. Judd has further supplied us with valuable 

 information on this very subject, the analogues of the Arran dykes 



