456 
THE CARBONIFEROUS VOLCANOES 
KOOK VI 
Where a sill has been injected among carbonaceous shales and coals it 
has undergone great alteration al'oug the contact, and if the sheet is only a 
few inches or feet thick, the change extends throughout its whole mass. 
Black liasalts and dolerites, in such circumstances, pass into a substance 
like a white or pale yellow clay, which at first might he mistaken for some 
band of fireclay intercalated among the other sediments. But evidence of 
actual intrusion may usually be found, as wlrere the igneous rock has caught 
up or broken through the adjacent strata, besides altering them. Such 
white traps, as they have been called, have long been familiar iir the coal- 
fields of Scotland and Central England. 
As a good illustration of the behaviour of such thin sills among 
carbonaceous shales I give here a section 
(Fig. 165) exposed in the old limestone 
quarry of Hillhouse, south of Linlithgow. 
At the bottom lies the Hurlet Limestone 
which has once been extensively mined 
at this locality. Abov’e it comes a 
group of black shale.s which in turn are 
surmounted by a sheet of beautifully 
columnar liasalt. The shales seem at 
first sight to include two layers of pale 
fireclay, each only a few inches in thick- 
Fig. 165.— Two thin sills of “Wiiite Traji ” ness. Closer inspection, however will 
Quarry, Linlithgow. are really sills which, thougil on the 
parallel with the bedding of the 
shale, may be seen to cut across it, and 
even at one place to send a finger into it. The upper example may also 
be obseived to diminish rapidly in thickness in one direction. 
The dimensions of the sills vary within tolerably wide limits. Although 
here and there the injected material dwindles down to an inch or less in 
thickness, running away even into threads, it more usually forms sheets of 
considerable depth. The rock of Salisbury Crags, for example, is fully 150 
feet thick at its maximum. That of Corstorphine Hill is probably about 
•350 feet. The great sheet which runs among the lower limestones from 
Kilsyth by Denny to Stirling has been bored through to a depth of 276 
feet, but as tlie bore started on the rock, and not in overlying strata, some 
addition may need to be made to that thickness. 
The spheroidal weathering so characteristic of basic eruptive rocks is 
nowhere more characteristically displayed than among the great doleritic 
sills of the liasin of the Firth of Forth. As an illustration of tliis structure 
an example is taken here from tlie large sheet at Korth Oueeusferry 
(Fig. 164). 
AVIiile one is struck with the great size and extent of some of the sills 
connected with the puys, as compared with tlie small and local sheets 
underneath tlie jdateaux, tliere is a further fact regarding them that 
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