VOLCANIC ROCKS OF EAST LOTHIAN. 121 



some a striped felspar is also developed (Phantassie Quarry, Skid Hill, Bangley Quarry). 

 In the Bangley Quarry so much plagioclase is present as to suggest a passage to the 

 andesites. The chemical analysis points the same way, the silica percentage being only 

 58*5. (See Chemical Analyses of the trachytes.) 



Structure of the Trachytes. — The noncrystalline character of the ground-mass, and 

 the wide breach between its constituent microlites and the porphyritic sanidine crystals, 

 have already been alluded to. One interesting feature in regard to the porphyritic 

 crystals remains to be noticed. There is, namely, a tendency in the latter to pack 

 themselves together, producing a glomero-porphyritic structure. This is especially 

 evident in the rocks of Kae Heughs, Dirlton Craig, and Peppercraig. The crystals are 

 so closely fused that the composite character of the glomero-porphyritic aggregates is only 

 noticeable between crossed nicols. The component grains then become distinct in con- 

 sequence of their different action on polarised light. One such mass, that appeared 

 homogeneous in ordinary light, was found to be made up of fifteen distinct grains when 

 examined between crossed nicols. In some cases the grains are allotriomorphic towards 

 the interior of the mass, but present idiomorphic contours at the exterior. The same 

 phenomenon has been observed by Mr Teall in the glomero-porphyritic felspar of the 

 Tynemouth Dyke. # 



With regard to the ground-mass, a beautiful micro-fluidal structure is produced in 

 many of the rocks {e.g., Dirlton Craig and Skid Hill) by the orientation of the microlitic 

 felspars of the ground-mass in lines which flow and eddy round the porphyritic crystals. 

 The interstices between the lath-shaped felspars are filled with allotriomorphic felspar and 

 with granules of augite : in no case was any glassy or felsitic base observed. Larger and 

 in some cases well-contoured crystals of a greenish augite sometimes occur. They usually 

 contain much enclosed magnetite. 



The Non-Porphyritic Trachytes. — Non-porphyritic varieties of the trachytes occur at 

 Score Hill, Lock Pit Hill, Craigie Hill, and Pencraig. They are pale rocks, tinted 

 variously with buff, pink, mauve, and cream colours. In texture they are compact and 

 " trachytic," and present a somewhat glistening appearance when subjected to minute 

 inspection. Examined under the microscope, they are seen to consist of a mass of close 

 interlacing lath-shaped crystals of a felspar which, between crossed nicols, gives no sign 

 of twin-striation. Scattered evenly through the sections are small patches of powdery 

 carbonate (calcite or dolomite), in some cases apparently pseudomorphous after augite. 

 Under a higher power the structure is seen to be completely, though minutely, crystalline, 

 the interspaces between the lath-shaped felspars being filled with granules of the same 

 mineral. Iron-ores are present in small quantity. A fluidal structure is occasionally 

 indicated by a parallel arrangement of the felspar microlites. 



In the salmon-pink rock of Lock Pit Hill the microlitic and long lath-shaped character 

 of the felspars is strongly pronounced, slender crystals of orthoclase, lying in a ground- 



* Some North of England Dykes, Quart. Journ. Geol. Soc, 1884, p. 234. 



