314 G.W. Tyrrell — Tertiary Dykes of the Clyde Area. 



still be referred to cumbraite, occurs near Cruach Chuilceachan, in 

 Cowal. This rock has been described by Sir J. J. H. Teall (in Clough, 

 1897, p. 155). It shows reticulations and bandings of varieties 

 differing in texture and amount of glass. It carries glomero- 

 porphyritic groups of basic felspar and hypersthene in a richly glassy 

 groundmass containing microlitic felspar and augite. 



The great Eskdalemuir (Dumfriesshire) dyke described by Sir A. 

 Geikie (1880, pp. 219-55; 1897, vol. ii, pp. 133 et seqq.) forms 

 a distinct variety of cumbraite distinguished texturally from the 

 Cumbrae rocks. In the latter both the felspars and pyroxenes of 

 the groundmass are slender and elongated, and may show a tendency 

 to subvariolitic groupings, as also in the leidleite and inninmorite of 

 Mull. In the Eskdalemuir type (Fig. \b) the felspars are much 

 broader, giving almost square or rhomboidal outlines, and the augite 

 tends to form small clusters of minute, granular, equidimensional 

 crystals. The enstatite stands out, however, as somewhat larger 

 prismatic crystals. The abundant glass is yellow and comparatively 

 free from microlites. Further differences are the comparative 

 paucity of the large phenocrysts of anorthite, and the presence of 

 sharply crystallized and uniformly distributed grains of magnetite in 

 the Eskdalemuir type. Sir A. Geikie has noted the occurrence of 

 curious enclosures of almost or quite holocrystalline material 

 consisting mainly of granular augite and felspar laths. The 

 " sheath-and-core " structure, of which this rock presents the type, 

 must also be mentioned, as it appears, more or less perfectly, in 

 many of the rocks treated in this paper. This rock may be 

 distinguished as the Eskdalemuir type of cumbraite. The contrast 

 between the textures of the two types is shown by Eigs. \a and \b 

 (see also Teall, 1888, pi. xiv, fig. 1). 



Kocks probably belonging to the Eskdalemuir type occur in the 

 four great W.N.W. dykes which traverse the Muirkirk district 

 of Ayrshire, the most southerly of which is the continuation of the 

 Eskdalemuir dyke. Enstatite, however, is not so abundant in these 

 dykes as in those described above, and is always represented by green 

 pseudomorphs. Furthermore, the anorthite phenocrysts are both 

 small and sparse, and may be locally absent. The yellow glass of the 

 groundmass provides abundant quartz on its devitrification. 



At least two dykes in the Cowal area belong to the Eskdalemuir 

 type and show the "sheath-and-core" structure, as described by 

 Clough (1897, pp. 135, 142). The Cruach Mhor dyke carries 

 a considerable amount of enstatite in small prisms. The dyke near 

 Brackleymore School shows enstatite in central intergrowth with 

 augite, but frequently decomposed to a fibrous green mineral 

 of straight extinction. A dyke from the burn half a mile west 

 of Loch na Leirg, Whiting Bay, Arran, possesses affinities with 

 this type. 



Typical cumbraites have not yet been found in Arran, but rocks 

 which may represent an almost holocrystalline development of the 

 cumbraite magma occur as north-west dykes penetrating the great 

 sill of teschenite or crinanite at Dippin (Tyrrell, 1916, pp. 193-6). 

 These rocks carry a few small phenocrysts of bytownite, frequently 



