O. W. Tyrrell — Alkaline Igneous Rocks, West Scotland. 69 



J, J. The line of junction between tachylite and stony rock is perfectly sharp. 

 It is seen apparently truncating a small felspar on the right-hand side, while 

 a curved lath is seen just below the contact near the centre of the figure. 

 The cracks in the lower layers are not well shown. 



Fig. 2. Shows a section cut roughly perpendicular to the upper part of Fig. 1. 



C, C. Cooling cracks, ch. The chlorite fibres bordering them. BG. Dark- 

 brown isotropic glass. Z. Zones of paler glass. F. Felspars often faulted 

 by cooling cracks. The direction of flow (L, L) is shown by the shape 

 and parallelism of the black cigar-shaped bubbles (b, b) ; some of these 

 are apparently hollow, while a yellow variety is shown at X. It is quite 

 evident that the cooling cracks here (C, C) have no connexion with the 

 direction of flow ; indeed, they are sometimes seen faulting the bubbles. 



IV. — The Late Palaeozoic Alkaline Igneous Rocks of the "West 



OF Scotland. 



By G. W. Tykkell, A.E.C.Sc, F.G.S., Assistant to the Professor of Geology, 

 Glasgow University. 



Introduction. 



OF late years it has become manifest that the igneous rocks of the 

 Carboniferous in Central Scotland have a distinct alkaline facies. 

 Such rocks as monchiquite, nepheline-basalt, mugearite, kulaite, 

 phonolite, essexite, and teschenite have been described from the 

 Lothians ; whilst in the western counties nepheline-phonolite, 

 theralite, mugearite, and teschenite are already known. There is 

 abundant evidence, however, that in the West the alkaline phase 

 is of later date than in the Lothians, and of late Carboniferous or 

 Permian age. In Arran it probably extended into the Triassic, 

 assuming that the stratigrapliy which assigns certain rocks to tha 

 Triassic is correct. A general account of this connected suite of 

 alkaline rocks, together with the lavas in the Mauchline Basin, to 

 which tliey can be shown to be genetically related, is given in this 

 paper, which deals only with rocks demonstrably later than the 

 volcanics of the Calciferous Sandstone. The work on which this 

 paper is based was commenced in 1908, and was assisted in 1909 by 

 a grant from the Government Grant Committee of the Koyal Society, 

 for which grateful acknowledgment is made. An unfortunate break- 

 down in health, however, necessitated the postponement of the 

 investigation for nearly a year. It is still incomplete owing to the 

 difficulty of obtaining adequate chemical analyses, without which it 

 is impossible accurately to determine the affinities of some of the 

 rocks. Pending the completion of the detailed work it has been 

 thought advisable to present this preliminary account of an igneous 

 suite of extraordinary variety and interest. 



Literature. 



A brief account of the literature has been given in an earlier paper. ^ 

 Allport described some of the rocks in his classic paper on " The 

 Carboniferous Dolerites".* Teall first recognized the teschenitic 



^ Trans. Glasgow Geol. Soc, vol. xiii, pt. iii, p. 300, 1909. 

 2 Q.J.G.S., vol. XXX, pp. 529-67, 1874. 



