VOLCANIC ROCKS OF EAST LOTHIAN. 



123 



Trachytic material builds up the plugs that form the Bass Eock and North Berwick 

 Law, while Traprain Law consists of an interesting trachytic phoDolite,* a type of rock 

 which does not appear to be represented elsewhere in the district, although it belongs to 

 the same petrographical family as the sanidine-trachytes of the Garltons. 



North Berwick Law.f — This shapely hill, which forms a prominent feature in the 

 scenery near North Berwick, is built up of a reddish brown rock of close texture, and 

 characterised by a curious glistening appearance. The microscope shows it to be a 

 trachyte. It is composed of a plexus of long and slender lath-shaped crystals of clear 

 felspar (sanidine), occasionally twinned on the Carlsbad type. The meshes between the 

 longer crystals are filled with a confused mass of minute spicules and microlites of the 

 same mineral. Beyond an indefinite ferruginous material, felspar is the only constituent 

 visible in the slide. 



The rock has been analysed by Mr J. S. Grant-Wilson for the Geological Survey of 

 Scotland, with the following result : — 



Si0 2 . 











60-15 



A1 2 B 



. 









18-04 



Fe 2 3 











4-44 



FeO . 



. 









1-82 



MnO . 











•13 



CaO . 











1-68 



MgO . 



. 









•98 



K 2 . 











4-15 



Na 2 . 











607 



H 2 . 











2-06 





99-52 





Sp. ( 



>. = 2-4€ 









It will be seen from this analysis that the composition of the rock is in close agree- 

 ment with the trachyte-flows (compare the analysis of the Peppercraig rock). In 

 petrographical habit it resembles some of the non-porphyritic trachytes of the district. 



The Bass Eock. — It was suggested by Sir Archibald Geikie that the Bass Rock was 

 one of the vents from which flowed the lavas of the Garlton Hills. I have been able to 

 substantiate the correctness of the suggestion by a petrographical study of material 

 obtained by Mr J. G. Goodchild during a recent visit to the island. The rock proves to 

 be a trachyte, similar in character to that of North Berwick Law, and to the non- 

 porphyritic division of the trachytic flows. The reddish-brown material is composed 

 almost exclusively of felspar (sanidine), the rectangular facets of which can be easily 



* The only phonolite that has hitherto been described in the British Isles is that of the Wolf Eock, off the coast 

 of Cornwall. 



t In the Survey Memoir on East Lothian (p. 51), North Berwick Law is described as "a round or slightly oval 

 plug of felstone, which comes up vertically through the ash, and when it reaches the surface of the ground tapers up 

 into a cone, of which the top is 612 feet above the sea. The rock on the higher part of the hill is a compact and finely 

 crystalline clinkstone, while further down it becomes more loose and granular in texture. At the foot of the cone, on 

 the west side, sandstone and black shale (strata, probably in the ashy series) are seen to dip away from the felstone at 

 an angle of 30°." 



VOL. XXXVII. PART I. (NO. 8). U 



