CHAP. XXVI 
MATERIALS ERUPTED BY THE PUYS 
419 
5. Basalts 
Finer-grained rooks, generally with a porpliyritic ingredient and niuoli scattered 
interstitial matter in the groundmass. 
5a. Porpliyritic felspar, and occasionally a little angite ^ 
CTvmmrlmaBa nf (rmruilai* niTait.p fplanav nppHlpB jnul -P.iiiiiv Hralcr ^Pvnp. 
Taking first the superficial lavas, I know of only one locality where 
picrite occurs in such a position that it may be included among the 
surface outflows. This is the quarry near Blackburn, to the east of Bath- 
gate, where I originally observed it.^ The rock occurs there on the line 
of the basalt-flows from the Bathgate Hills, and 1 mapiped it as one of them 
before the microscope revealed the remarkable composition of the mass. I 
still believe it to be a lava like the “ leckstone ” described on p. 443, though 
the other known examples of this rock in tlie basin of the Firth of Forth 
are intrusive sheets. The rock locally known as “ leckstone ” or “ lakestone ” 
has long been quarried for the purpose of constructing the soles of bakers’ 
ovens, as it stands a considerable temperature without cracking. Its micro- 
scopic structure is now well known. As exposed in Blackburn quarry, an 
interesting difference is observable between the lower and upper parts of the 
sheet. The lower portion is a picrite, with abundant serpentinized olivine, 
large crystals of angite, and a considerable amount of ores. The upper 
portion, on the other hand, has plagioclase as its most abundant definite 
mineral, with a minor quantity of minute prisms of augite and of iron-ores, 
and scattered crystals of olivine. Here, within the compass of a few yarils 
and in one continuous mass of rock, we have a transition from a variety of 
olivine-basalt into a picrite. 
The great majority of the puy lavas belong to the olivine-bearing series. 
A few of them are dolerites, but most are true basalts of the Hahueuy type, 
of which typical examples may be seen at the Ivirkton quarries, Bathgate, 
and in the coast section between Pettycnr and Kiughorn. Occasionally 
they present transitions towards picrite, as in the sheet overlying the lowest 
limestone at Kirkton, and in the lowest lava of King Alexander’s Crag, 
Burntisland. These puy lavas exhibit considerable variety of structure as 
seen in the field. Some are solid, compact, black rocks, not infrequently 
columnar and weathering into spheroidal exfoliating forms. Others are 
somewhat granular in texture, acquiring green and brown tints by weather- 
ing, often showing amygdaloidal kernels, and even passing into well-marked 
amygdaloids. ilany of them exhibit a slaggy structure at their upper and 
under surfaces (Figs. 153, 170, l7l). These external differences are an 
’ Trans, lluy. Soc. Edin. voL xxix. (1879) p. 506. 
