CHAP. XXVI 
MATERIALS ERUPTED BY THE BUYS 
421 
.sometimes lie a mere accident of cooling or otherwise. I have .sliown that 
ill the .same mass of rock at Illackhnrn a gradation can he traced from a 
rock largely composed of altered olivine into one consisting mainly of felspar 
with bnt little olivine, and another example occurs in the picrite-sill between 
Edinburgh and Eramond. Dr. Stecher lias ascertained that the marginal 
portions of the sills in the basin of the Firth of Forth, which cooled first and 
rapidly, and may be taken, therefore, to indicate the mineral composition of 
the rock at the time of e.xtrusion, are often rich in olivine, while that 
mineral may be hardly or not at all discernible in the main body of the rock.^ 
(3f the ordinary and characteristic dolerites without olivine which con- 
stitute most of the intrusive masses, the various types enumerated in the 
tabular arrangement are almndantly developed in Central Scotland. Thus 
the normal ophitic type is displayed by the uppermost sill of the Burnt- 
island series, and by the rock which forms the plug of the Binns Flill neck 
in Linlithgowshire. The Eatho type is well seen in the large sill at Eatho, 
likewise in the extensive intrusive sheets in the west of Linhthgowsliire as 
at Muckraw and Cairibher. Tlie Burntisland sill type is shown by the 
lower sills of Burntisland and by some others in the same region, especially 
by that of Colinswell, and liy another on the shore east from tire Poorhouse, 
near Kinghorn. The great boss among the Bathgate Hills likewise displays 
it. The Bowden Hill type occurs in well-marked development at Bowden 
Hill, tliree miles south-west of Linlithgow, and in the massive sill at St. 
IMargaret’s, west from Xorth Queensferry. 
The non-olivine-bearing basalts are found in various bosses aud sheets 
in the basin of the Firth of Forth. Thus the Biniiy Craig type occurs in 
the jirominent and pieturesq^ue sill from which it is named, likewise among 
the intrusive sheets near’ Kirkcaldy, in FTfe. Sometimes the same mass ol 
rock displays more than one type of structure, as in the case of the great 
(lalabraes neck among the Bathgate Hills wherein both the Tholeiite and 
Burntisland sill types may be recognized. 
Some of the sills in West Lothian, as I pointed out many years ago, 
contain bitumen and give off a bituminous odour when freshly broken. 
They have been injected into bituminous shales or coal-seams.'^ 
2. Andesites. — Eocks referable to this series ajipear to have been of 
rare occuri’ence among the puy-eruptions. Examples of them containing as 
much as 60 per cent of silica occur among the lavas of the Limerick basin. 
Some of the necks and what may be sills in the same district likewise 
consist of them. 
o. Trachytes and Quartz -bearing Eocks. — Acid rocks, as I have 
already said, are extremely rare among the puy-eruptions. The only im- 
portant exanqiles known to me are those around the Limerick basin, where 
they rise apparently in old vents and form conspicuous rounded or conical 
hills. These rocks have been examined microscopically by Mr. W. W. 
1 Dr. Steelier, 'MiermaPs Minemlog. MUthri/. vol. ix. (1887) p. 193. Proc. Hoy. Soc. Min. 
vol. XV. (1888) p. 162. 
Geol. Survey Memoir on Geology of Bdinburgli (Sheet 32, Scotland), p. 46. 
