G. W. Tyrrell — Alhaline Igneous Rocks, West Scotland. 129 



characterized respectively by nepheline and leucite. jSTepheline is of 

 comparatively rare occurrence. There is abundant evidence in these 

 rocks that it can be formed along with primary analcite. The 

 dominant felspar is labradorite of the composition Ab;^ An^^ ; orthoclase 

 (a soda-bearing variety or sometimes anorthoclase) occurs in very 

 subordinate quantity. 



The characteristic ferro-magnesian mineral is a pleochroic augite of 

 a peculiar deep purple-madder tint, which is generally held to indicate 

 a high titanium and soda content. Olivine is very abundant, and a 

 peculiar red barkevicitic hornblende is occasionally present in some 

 quantity, ^girine and arfvedsonite appear in the more acid rocks. 



The suite has a general basic character. Amongst the intrusive 

 rocks teschenite is by far the most abundant. It shows a decided 

 tendency to pass over to ultra-basic modifications. Subacid rocks are 

 very rare in comparison, and acid rocks are unknown. The lavas are 

 basic, with a large number of types verging on the ultra-basic. The 

 abundance of analcite indicates a parent-magma rich in water and 

 alkalies. Other volatile substances were probably present which are 

 not now represented among the constituents of the rocks. Apatite, 

 however, is very abundant, and seems always to be associated with 

 analcite or alkali -felspars. Hence it is probable that fluxes like 

 fluorine or chlorine were present in the magma. Whatever fluxes 

 were present, there is evidence to show that the magmas were usually 

 very liquid ; and to that liquidity must be ascribed the very complete 

 differentiation seen in some of the intrusions. The liquidity in some 

 cases was so great that differentiation was controlled more by simple 

 gravity-stratification than by differences of temperature set up during 

 intrusion. This remark applies to differentiation after intrusion ; 

 but assuming a single original magma-reservoir, there can be little 

 doubt that some differentiation took place before intrusion, as is 

 evidenced by the variety of rocks which constitute the suite. 



DlSTRIBUTIOlS' AND AgE. 



The areal distribution of the various types seems to have some 

 significance. The teschenites are the most widely spread types. 

 They are mostly intrusive in the Carboniferous Limestone Series in 

 the areas remote from the central volcanic districts of Mauchline and 

 Dalmellington. Thus they are common in ]S^oi"th Ayrshire and in the 

 Cumnock districts. The Glasgow teschenites are separated from the 

 nearest Ayrshire occurrences by a wide interval, which is mainly 

 occupied by a great mass of calcifei'ous sandstone volcanics. If they 

 are connected with the Ayrshire centre it is remarkable that no 

 teschenites are to be found in the intervening area ; unless it is that 

 these rocks found the calciferous sandstone lavas difiicult of penetration. 



The principal teschenite-picrite masses are ranged on the margin of 

 the Coal-measure area surrounding the central volcanic district, as at 

 Cumnock, Lugar, and Patna. The kylites penetrate the Coal- 

 measures in the zone immediately surrounding the ' Permian ' lavas 

 of Mauchline, and are mostly concentrated in the southern part of 

 that zone. The essexite-dolerites and analcite-syenites intrude the 

 lavas and overlying sandstones of the Mauchline area ; and a zone of 



DECADE V. — VOL. IX. — NO. III. 9 



