CHAP.. XXIV 
materials of the plateau-eruptions 
379 
at Markle quarry the rock is still more felspathic and contains the olivine 
only in small sporadic grains. The composition ol these basic rocks of t le 
Garleton plateau is shown in the subjoined table of analyses by Mr. . . 
Grant Wilson '; — 
SiO-j 
Al^Oa 
FeoOs 
FeO 
MuO 
CaO 
ilgO 
Na20 
HoC) 
Total I 
Kippie Law, specific 
gravity 2*8 
46-01 
19-19 
5-91 
6-75 
0-19 
8-68 
6-81 
1-20 
3-27 
3-07 
101-08 
Hailes Castle, specific 
fjravity 2-76 . 
49-07 
19-43 
10-58 
2-35 
0-32 
7-87 
4-36 
0-98 
3-31 
2-26 
100-53 
Markle Quarry, specific 
gravity 2 ’7 
49-54 
22-23 
9-55 
1*12 
0-08 
7*19 
2-80 
1-81 
4-56 
2-42 
101-30 
Olivine-dolerites are more especially developed in the district around 
Jedburcdi, where they form some of the most prominent bosses, such as 
Guniair and Black Law. They show a suhophitic groundmass, with incon- 
spicuous porphyritic crystals, among which those of olivine are more promi- 
nent than the felspars (p. 418). , , ^ , f 
(r) Andesites (Porphvrites).— These are the most abundant lavas *ot 
the plateaux. They occur in every district, and usually form the mam 
constituents of the pile of volcanic material. They vary m colour from a 
pale pinkish grey, tlirough many shades of red, purple, brown and yellow, 
to sometimes "a dark green or nearly black rock. Their texture ranges 
from almost semi- vitreous, through different degrees of compactness, to open, 
cellular, slaggy masses. Generally through their base porphyritic felspars 
are abundantly disseminated, sometimes in large, Hat, tabular forms, like 
those of the Lower Old Bed Sandstone already referred to. The amyg- 
daloidal kernels consist of calcite, zeolites, chalcedony or quartz. It is 
from the amygdaloids on either side of the Clyde that the fine examples o 
zeolites have been chiefly obtained for which the south of Scotland has long- 
been famed. Occasionally, as at the south end of Bute, the audesitic lavas 
display a marked columnar structure. . , , . 
Under the microscope these rocks present the usual fine felted aggrega e 
of felspar microlites, with granules or crystals of magnetite and sometimes 
pyroxene. The porphyritic felspars, often large and well defined, generally 
contain inclusions of the groundmass. Occasionally some of the large por- 
phyritic constituents are augite, or pseiidomorphs after that mmeial. _ le 
deration of the rocks has oxidized some of the iron-ore and given rise to 
the prevalent purplish and reddish tints. , , , , .u i i- i • 
(cl) Trachytes. Some of the most remarkable lavas to be found in 
any of the plateaux are those which constitute a large part of the Garleton 
Hills. They overlie the lower andesite and basalt platform, which surrounds 
them as a narrow belt, while they occupy the central and much the largest 
part of the area. They have been included among the porphyrites, but are 
pale rocks, generally with a yellowish crust, presenting when quite fresh a 
orey, compact, felsitic base with large porphyritic crystals of iinstriped felspar. 
