CHAP. XXIV 
MATERIALS OF THE PLATEAU-ERUPTIONS 
381 
of a liglit green soda-augite, with practically no magnesia in it. A small 
cniantity of iron-ore and isolated granules of apatite are also present, to- 
gether with patches of nepheline which, though generally decomposed and 
replaced with zeolitic products, occasionally display six- and four-sided 
crystal-contours. An analysis of the Traprain phonolite by Mr. Player is 
subjoined : — ^ 
Silica ■ 
Titanic acid 
Alumina . 
Ferric oxide . 
Ferrous oxide 
Manganous oxide 
Lime 
Magnesia . 
Soda 
Potash 
Loss by ignition 
99-4 
Spec. grav. 2-588 
Tlie neck of North Berwick Law was found by Dr. Hatch to he^ a 
trachyte, showing a plexus of lath -shaped sanidines that diminish in size 
to minute mierolites, but with no porphyritic or ferro- magnesian con- 
stituent. The Bass Bock, though its geological relations are concealed by 
the sea, is in all probability another neck of this district. It is likewise a 
mass of trachyte, composed almost entirely of lath-shaped crystals of sanidine, 
with no ferro-magnesian constituent, but a good deal of iron ore. It shows 
none of the large porphyritic felspars so characteristic of the Garleton Hills 
lavas but it closely resembles the non-porphyritic varieties, particularly the 
lavas of Score Hill, Pencraig, Lock Pit Hill, and Craigie Hill.^ 
(/) Tuffs. The fragmentary ejections ot the plateaux vary in texture 
from the finest-grained tuffs to coarse agglomerates.® As they have been 
derived from the explosion of andesite-lavas, they consist mainly of the debris 
of these rocks. They are often deep red in colour, as for example those of 
Dunbar, but are most frequently greenish. They have a granular texture, 
due to the small lapilli of various porphyrites imbedded in a fine dust of 
the same material. Grains of quartz, frequently to be detected even in 
the finer tuffs, may either have been ejected from the volcanic vents, or 
may have been grains of sand in the ordinary sediment of the sea- 
bottom Both at the base and at the top of the plateau-senes, the tuffs are 
interstratified with and blend into sandstones and shales, so that specimens 
may be collected showing a gradual passage from volcanic into non-volcanic 
detritus In many of the tuffs of the necks fragments of sandstone and 
0-5 
19-7 
2-2 
3- 5 
0-2 
2-2 
0-4 
4- 3 
7-1 
2-5 
1 Trans. Hoy, Soc. Edin. vol. xxx-vii. p. 125. , , „ 11 1 1 « f 
» The composition of the rocks of North Berwick Law and the Bass closely resembles that of 
the trachytic lavas of the platean. For analyses, see Dr. Hatch’s Paper, -ibid pp. 123, 124. 
For accounts of these rocks, see Explanation of Sheet 33 Geol. Surv. Scot. p. 32 ; Sheet 22, 
pp. 11-14 ; Sheet 31 pp. 14-17. 
