308 G. W. Tyrrell — Tertiary Dykes 



(Ab x An^. The contact between the two felspars is remarkably 

 sharp, giving a well-marked "bright line" effect. The marginal 

 labradorite has the same width whatever the size of the phenocryst, 

 and has a very ragged, crenulated, or spiky margin against the glass 

 of the groundmass, as though the glass had penetrated along the 

 cleavage cracks. The anorthite phenocrysts carry numerous 

 globulitic inclusions and " negative crystals " of dark glass extended 

 parallel to 010. These are zonally arranged or confined to the 

 centres of the crystals. The groundmass shows labradorite laths 

 and pyroxene prisms embedded in a dark glass, the crystalline and 

 glassy matter being about equal in bulk. The felspars form 

 elongated laths, with bifid or trifid terminations, and a maximum 

 extinction anjjle of 28 degrees, indicating acid labradorite 

 {Abi An^. The chemical analysis shows that the orthoclase 

 molecule must be present, but it does not appear in crystalline form, 

 and must be incorporated in the glass. 



The pyroxene includes both enstatite and augite, which are 

 ■usually intergrown in characteristic fashion, with the augite always 

 occurring on the margins. A common appearance is of three prisms 

 in parallel position, two of augite on the margins, and a stout crystal 

 •of enstatite occupying the centre. Frequently, however, the enstatite 

 •occurs without the flanking augites, especially in the other Cumbrae 

 dykes. It then carries minute specks of magnetite in a marginal 

 zone. It is quite fresh, colourless, with straight extinction, 

 positive sign, and has the heavily outlined cross-fracturing often 

 seen in the orthorhombic pyroxenes. The augite is also colourless 

 and has an extinction of 36 degrees. 



The apparently dark-brown or black glass of the groundmass may 

 be resolved under a high-power objective into a colourless or pale- 

 yellow glass darkened by innumerable black globulites. It also 

 carries numerous curved felspar microlites, and little strings 

 of iron-ore granules which may represent original microlites of 

 pyroxene. The apparently more glassy part of the Eerie Port 

 dyke differs from the stonier part only in the greater freshness of 

 its glass. The glass of the stony part always shows the beginnings 

 of devitrification. 



The other dykes of this type in the Great Cumbrae differ from the 

 Eerie Port dyke only in minor details, such as the relative abundance 

 of phenocrysts or the glassy base. 



Mode. — An attempt was made to determine the quantitative 

 relations of the minerals in the Eerie Port dyke by the llosiwal 

 method, but owing to the presence of glass, the fine grain, and the 

 consequent uncertainty as to the exact boundaries of the minerals, 

 the results must be regarded as only approximate. The anorthite 

 phenocrysts were estimated from a number of hand-specimens. 

 Table I shows the results of measurements for the "stony" (1) and 

 " glassy " (2) parts of the Eerie Port dyke. 



These figures show that this rock, if holocrystalline, would have 

 been dofelsic. The glassy material contains little or no ferro- 

 magnesian constituents, as shown later. As regai'ds texture, the rock 

 ismegaphyric (large phenocrysts), hyalocrystalline (glass and crystals 



