of the Clyde Area. 307 



adjacent to the Marine Biological Station at Millport attains a height 

 of 30 feet above the raised beach, and averages 20 feet in width. 

 The exposure from which the type-rock has been taken occurs on the 

 raised beach at Eerie Port on the north-west side of the island. This 

 dyke is 29 feet wide, has a N.N."W. trend, and a slight hade to the 

 W.S.W. Its central part has locally a more vitreous lustre or 

 pitchstone-like appearance than other parts. Petrographical examina- 

 tion shows that this appearance is not due to greater richness in 

 glass, but to its slightly fresher character. All parts of the dyke 

 show small cavities partly or wholly filled with bitumen. This 

 dyke can be traced inland for about a mile, which is about as far as 

 the Lion Rock and the Marine Biological Station, where dykes can 

 also be traced. Other and smaller dykes of the same character occur 

 around the shores of the island. 



Pig. la. — Cumbraite, N.N.W. dyke, Eerie Port, Great Cumbrae. x 20. Large 

 phenocryst of anorthite ; groundmass : laths of labradorite, ehstatite, 

 augite, in base of dark glass. 

 ,, 16.— Cumbraite (Eskdalemuir type), W.N.W. dyke, Waterhead Burn, 

 Muirkirk, Ayrshire. x 20. Microphenocrysts of labradorite and 

 magnetite ; groundmass : felspar microlites, grains of augite and 

 enstatite, in base of dark glass. 



Under the microscope (Fig. la) these rocks show phenocrysts 

 of plagioclase near anorthite, in a groundmass of laths of acid 

 labradorite (Abi Am), enstatite, augite, and abundant glass. The 

 phenocrysts are extraordinarily fresh, with both Carlsbad and albite 

 twinning usually well developed. They may reach a diameter of 1 cm. 

 Twinning, cleavage, and refractive index methods agree in deter- 

 mining the composition of the felspar as between Abis An^ and 

 Ab 10 An 90 , i.e. anorthite near bytownite. The mineral, moreover, 

 has a negative optical character which agrees with this determination. 

 All the crystals have a narrow border of optically positive labradorite 



