ANKIVEESAEY ADDRESS OF THE PEESIDENT, 1 63 



volcanic phenomena in Tertiary time/ I need not do more on the 

 present occasion than submit a mere summary of the subject. 



The Tertiary volcanic rocks of this country display in their modes 

 of occurrence some marked contrasts to those of older geological 

 periods. The first of these to attract the attention of the student is 

 the important part played by dykes, and the enormous area over 

 which these are distributed. The second point of difference lies in 

 the exceedingly restricted area of the superficial lavas now visible 

 compared with the wide extension of the dykes. Further contrasts 

 are to be found in the well-bedded structure of these lavas ; their 

 disposition in horizontal or nearly horizontal sheets, forming plateaux 

 far broader and better defined than those of the Carboniferous 

 system ; the scarcity or absence of visible vents from large tracts 

 over which the lavas spread; the non-appearance of marine sediments 

 in any part of the volcanic series, and on the contrary the presence of 

 remains of terrestrial vegetation in such positions as to indicate that 

 the eruptions took place on land ; and lastly the want of any imme- 

 diately succeeding conformable formation which might have served 

 to chronicle the close of the volcanic eruptions, and thus to define 

 their upward geological limit. 



On the other hand, the essential features of volcanic action 

 are repeated with the same regularity in the Tertiary as in the 

 older periods. We see that basic lavas flowed out first, that acid 

 protrusions came later, that the vents of eruption were made use 

 of for the uprise of huge bosses and plugs of different molten 

 rocks, and that an abundant series of sills has been injected into 

 the platform on which the bedded basalts lie. 



Taking first the dykes as the most singular and widespread 

 feature of the Tertiary volcanic period, we find them to be distributed 

 over a region of somewhere about 40,000 square miles, that is more 

 than a third of the total land-surface of the British Isles. They 

 are more particularly developed in the west, centre, and south of 

 Scotland, the north of England, and the north of Ireland. Within 

 this wide region there are doubtless many dykes much older than 

 those which alone can be classed as Tertiary. Excluding these, we 

 nevertheless find many thousands which are united by certain 



^ ' The History of Volcanic Action during the Tertiary Period in the British 

 Isles,' Trans. Eoy, Soc. Edin. vol. xxxv. (1888) p. 21. I may refer to this paper 

 for the previous literature of the subject. Since its publication, Prof. Judd, 

 with whose ideas as to the order of succession of the volcanic phenomena of the 

 luner Hebrides I do not agree, has published a criticism of it in Quart. Journ. 

 Geol. Soc. vol. xlv. (1889) p. 187, and a further paper in vol. xlvi. (1890) p. 341. 



