ANCIENT VOLCANIC ROCKS. 53 



by the general field-term " porphyrite ". Judging from the 

 variety of rock-types recognised by the examination of the 

 small plateau of the Garlton Hills in East Lothian, it seems 

 probable, however, that the whole group will be found to 

 yield many results of interest. 



Some of these Garlton Hills rocks have been described 

 by Hatch (3). They are partly from lava-flows, partly from 

 " necks " which are believed to mark the actual sites of 

 volcanic vents. In some cases the rocks have retained 

 their original characters with remarkable freshness. The 

 lower lavas are of thoroughly basic composition, and, for the 

 most part, are olivine-basalts with from 46 to 49^- per cent, 

 of silica. One, however, is of ultrabasic nature, with only 

 40 per cent, of silica and large proportions of lime, magnesia, 

 and lerrous oxide. Microscopical examination shows it to 

 be a well-marked type of limburgite or magma-basalt, com- 

 parable with those of the Kaisertuhl and other districts. 

 Felspar is unrepresented, except by an occasional skeleton- 

 crystal. The olivine is often quite fresh. The augite is 

 a titaniferous variety, showing the characteristic violet tint 

 and pleochroism. The crystals of these minerals, with some 

 magnetite, lie in a oround-mass consistino- of augite-micro- 

 lites and glassy matter, and some nepheline is probably 

 present. 



The upper lavas are equally interesting, being chiefly 

 unaltered trachytes with 58^ to 62J5 per cent, of silica and 

 10 per cent, of alkalies. They are holocrystalline rocks, 

 sometimes markedly porphyritic, and consist essentially of 

 fresh felspars with a small proportion of a green soda- 

 bearing augite or segirine. They are indistinguishable from 

 many Tertiary ajgirine-trachytes. Some of the volcanic 

 necks in the district, such as North Berwick Law and the 

 Bass Rock, are formed of trachytes not materially different 

 from those of the flows ; but one of them, Traprain Law, 

 shows an interesting difference. It consists mainly of little 

 sanidine-prisms with crystals of a bright green soda-augite, 

 but there are also little patches which close examination 

 proves to consist of nepheline and its alteration-products. 

 The rock is therefore a phonolite, though of a type 



