128 G. W. Tyrrell — Alkaline Igneonn Hocks, West Scotland. 



origin. The material is deficient especially as regards the chemical 

 analyses which are so vital to such a discussion. Nevertheless it is 

 possible to indicate broadly the salient characters of the suite. 



Oeological occurrence. — The rocks occur as sills, small lenticular 

 intrusive masses, volcanic plugs, and as a series of lava-flows. No 

 occurrence of true abyssal habit is known. The intrusions are on 

 a comparativelj' small scale, and range from occurrences, the outcrops 

 of which cover only a few hundred square yards, to some, such as 

 those of Craigie and Dundonald, which cover several square miles, and 

 have a thickness of over 100 feet. Many are lenticular in form; 

 others, however, ai"e stratiform, and have a horizontal extension 

 which is very great as compared with their thickness. This form 

 frequently characterizes the sills rich in aualcite, the original magma 

 of which must have possessed unusual liquidity. A concomitant 

 effect is a marked differentiation, which is usually best developed in 

 sills, such as that of Lugar, having a wide horizontal extension. 



The volcanic plugs may consist merely of lavaform material ; but 

 kylite, teschenite, and other rocks have also been found with this 

 mode of occurrence. At Carskeoch Hill, near Patna, the agglomerate 

 is pierced by a very heterogeneous complex (p. 120). The lavas occur 

 in thin im persistent flows. The 280 feet of thickness estimated by 

 Mr. Ferguson includes a large number of intercalations of tuff and 

 sandstone. The lavas have been poured out from several ' greenhill ' 

 vents, characterized by their bright-green vegetation and sharply 

 conical form. A tliin skin of lava is occasionally found adhering to 

 the walls of an agglomerate vent. 



The sieve-like perforation of the sedimentary rocks by these vents 

 is reminiscent of the Mid-Miocene vulcanicity of the Suabian Alb, as 

 remarked by Daly. It is noteworthy that Daly has adduced the 

 Ayrsliire and Fifeshire districts as examples of secondary vulcanism, 

 due to the opening of vents above satellitic intrusions. He remarks: 

 " The steady association of tuff-neck and sill in the Scottish shires 

 scarcely looks accidental." ^ 



A remarkable absence of dykes belonging to this suite is to be 

 noted. The only one known to the writer is a thick dyke-like mass 

 of theralite (Eellow type) crossing the Lugar Water just below the 

 Lugar sill. This is probalaly connected underground with the theralite 

 of the Lugar sill. The Crawfordjolin dyke is a doubtful example 

 and requires further investigation. 



Chemical and Mineralogical Characters. — The chemical characters of 

 the suite cannot as yet be adequately discussed. The chief points to 

 be noted are that tlie magmas generally are poor in silica and rich in 

 alkalies, especially soda, which is always in excess of potash. 



The characteristic mineral of the suite is aualcite. It occurs in 

 almost every member, sometimes in large quantity, and in such 

 relations to the other constituents as to establish indubitably its 

 primary character. Indeed, this suite should go far to demonstrate 

 the existence of an aualcite series of igneous rocks parallel to those 



' K. A. Daly, "Nature of Volcanic Action " : Proc. Amer. Acad. Arts and 

 Sciences, vol. xlvii, pp. 117-18, 1911. 



