CHAP. XIX 
THE OCHIL AND SIDLA IV ERUPTIONS 
309 
of the volcanic serie.s of the Ochil Hills are to he 
observed at the west end of the chain. But as 
the whole succession of rocks cannot conveniently 
be obtained along one line, it is better to make 
several traverses, starting in each case from a known 
horizon. In this way, by means of three parallel 
sections, we may obtain the whole series of lavas 
and tuffs in continuous order. The first line ot 
section starts in the lowest part ot the tulfs 
represented at the bottom of the group in lig. 
80, and runs up to the first thick ashy intercala- 
tion among the lavas. Following this bed south- 
westward to the Burn of Sorrow, we make from 
tliat horizon a second traverse across the strike 
to the summit of King’s Seat Hill (2111 feet 
above the sea), where we meet with a well-marked 
lava which can be traced south-westwards, gradu- 
ally descending the southern escarpment of the 
hills until it reaches the boundary fault near the 
village of Menstrie. Starting again from this 
definite horizon, we take a third line across the 
top of Huniyat (1373 feet) to the plain of Sheriff- 
muir, and there pass beyond the volcanic series 
into the overlying red sandstones. Arranged thus 
in continuous vertical sequence the succession is 
found to be a,s represented in Fig. 81. Tlie total 
thickness of volcanic material amounts to more 
than GoOO feet. 
In this vast pile of volcanic ejections the 
lavas are almost entirely andesites ot the usual 
characters. They include many slaggy and 
amygdaloidal varieties, some beautiful porphyries 
with large tabular felspars, likewise tlie resin- 
ous or glassy variety already referred to as 
occurring above Airthrey Castle. Their upper 
and under surfaces show the same structure as 
already described in those of the coast-sections in 
the Montrose tract. They include also more acid 
lavas, like the pale pink decom]3osing felsites ol 
the Pentland Hills. 
The tuffs and conglomerates occirr on many 
platforms throughout the succession ot lava-sheets. 
They -form the lowest visible part of the whole 
volcanic series, but tliey are most abundant towards 
the top, and are best displayed at the western 
end of the hills. In Dumyat they form a con- 
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