6 4 
THE TERTIARY VOLCANOES 
BOOK VIII 
Fig. 255.- 
-Section of coal rendered columnar by intrusive 
basalt, shore, Saltcoats, Ayrshire. 
a, Fireclay ; b, Coal rendered prismatic near the basalt ; 
e, Dark shale ; d, Basalt-rock. 
has been superinduced on carbonaceous strata, particularly on seams 
of coal. In the Ayrshire coal-field the alteration of the coal extends 
sometimes 150 feet from the dyke, the extent of the change depending 
not merely on the mass of the igneous rock, but on the nature of the 
coal, and possibly on other causes. 
Close to a dyke, coal passes into 
a kind of soot or cinder, some- 
times assumes the form of a 
finely columnar coke (Fig. 255), 
and occasionally has become 
vesicular after being fused. 1 
Shales are converted into a 
hard flinty substance that breaks 
with a conchoidal fracture and 
rings under the hammer. Fire- 
clay is baked into a porcelain- 
like material. Limestone is 
changed for a few inches into 
marble. As an illustration of this alteration, I may cite a dyke ten feet 
broad which cuts through the chalk in the Templepatrick Quarry, Antrim. 
For about six inches from the. igneous rock the chalk has passed into a finely 
saccliaroid condition, and its organisms are effaced. But beyond that distance 
the crystalline structure rapidly dies away, the micro-organisms begin to 
make their appearance, and within a space of one foot from the dyke the 
chalk assumes its ordinary character. 
Sandstones are indurated by dykes into a kind of quartzite, sometimes 
assume a columnar structure (the columns being directed away from the 
dyke-walls), and for several feet or yards have their yellow or red colours 
bleached out of them. The granite of Ben Cruachan where quarried on 
Loch Awe, as I am informed by Mr. J. S. Grant Wilson, is traversed by 
a basic dyke, and for a distance of about 20 feet is rendered darker in 
colour, becomes granular, and cannot be polished and made saleable. 
Where many dykes have been crowded together, their collective effects 
in the alteration of the strata traversed by them have sometimes been 
strongly developed. One of the most remarkable illustrations of this 
influence is presented by the district of Strathaird, which was cited by 
Macculloch for the abundance of its dykes. In recently mapping this 
ground for the Geological Survey, Mr. Ilarker has observed in some places a 
score or more dykes in actual juxtaposition, while over considerable dis- 
tances he found it difficult to detect any trace of the Jurassic strata, through 
which the igneous rocks have ascended. As might be expected under these 
circumstances, such portions of the strata as can be seen display an alto- 
gether exceptional amount of contact -metamorphism. Mr. Harker has 
noticed some limestones at Camasunary which have been changed into very 
remarkable lime-silicate rocks, with singular bunches of diopside crystals. 
1 Explanation of Sheet 22, Geological Survey of Scotland, p. 26. 
