ANNIVERSARY ADDRESS OF THE PRESIDENT. 



73 



rocks, like the nodular rhyolites of Bala and Upper Silurian age ; ^ 

 on the other hand, they seldom include such basic lavas as the 

 basalts, so common in the Carboniferous system, and never, so far 

 as I know, contain varieties comparable to the 'ultra-basic' com- 

 pounds which I shall have occasion to allude to as characteristic of 

 a particular volcanic zone in that system. 



(a) The Diabase-lavas are typically developed in the chain of the 

 Pentland Hills, where they form long bands intercalated between 

 felsitic tuffs, — a remarkable association, to which I shall make more 

 detailed reference in a later part of this Address. They range 

 in texture from a compact dark greenish base to a dull earthy 

 amygdaloid. One of their most remarkable varieties is a fine-grained 

 green porphyry, with large flat tabular crystals of plagioclase 

 arranged parallel to the direction of flow (Carnethy Hill). Most of 

 them, however, are more or less amygdaloidal, and some of them 

 (Warklaw Hill) strongly so. The following analyses, made many 

 years ago in the laboratory of the Eoyal School of Mines under the 

 direction of Prof. E. Prankland, but never published, show their 

 chemical comj)ositiou : — 



Analyses of Biahase-Lavas from the Pentland Hills. 





SiOo. 



AI2O3. 



FeA- 



FeO. 



CaO. 



MgO. 



KoO. 



NaaO. 



H^O. 



P2O5. 



0*48 



COo. 



1 



Carnethy Hill * 



51-16 



22-27 



1-30 

 17-46 



2-94 



4*02 



5-6I 



3-46 



2*42 



2-58 



3-42 



1-28 



r>, • 1 I Soluble in 

 2^^ (in ditto. 



52-00 



1-53 



7-85 



1-14 



2-43 

 6-80 



0-98 

 1-06 



1-66 



4-17 



2-68 



0-32 





Warklaw / Soluble in 

 Hill. ) HCl 



Sp. grav. j Insoluble 



2tl. (. in ditto. 47-77 



5-23 

 13-08 



7-32 

 0-84 



... 



7-88 

 4-07 



3-65 

 0-30 



1-17 



2-30 



2-48 



0-12 



5-01 



* There was a trace of manganous oxide in this specimen. 



(6) The PoRPHiRiTES, which constitute by far the largest propor- 

 tion of the lavas, have a characteristic but limited range of lithological 

 varieties. The prevailing type presents a close-grained, rather dull 

 texture, and a colour varying from pinkish grey, through many shades 

 of green and brown, to purplish red, which last is, on the whole, the 

 predominant hue. Minute lath-shaped felspars may frequently be 

 detected with the naked eye on fresh surfaces, while scattered crystals. 



^ The only examples known to me are those of Benaun More and other bills 

 in County Kerry. 



