516 
ELEMENTS OF GEOLOGY. 
Fig. 595. 
Basaltic dikes in chalk in Island of Rathlin, Antrim. Ground-plan as seen 
on the beach. (Conybeare and Buckland.*) 
The annexed drawing (Fig. 595) represents three basaltic 
dikes traversing the chalk, all within the distance of 90 feet. 
The chalk contiguous to the two outer dikes is converted 
into a finely granular marble, m, m, as are the whole of the 
masses between the outer dikes and the central one. The 
entire contrast in the composition and color of the intrusive 
and invaded rocks, in these cases, renders the phenomena 
peculiarly clear and interesting. Another of the dikes of the 
north-east of Ireland has converted a mass of red sandstone 
into hornstone. By another, the shale of the coal-measures 
has been indurated, assuming the character of flinty slate; 
and in another place the slate-clay of the lias has been 
changed into flinty slate, which still retains numerous im¬ 
pressions of ammonites.f 
It might have been anticipated that beds of coal would, 
from their combustible nature, be aflected in an extraor¬ 
dinary degree by the contact of melted rock. Accordingly, 
one of the greenstone dikes of Antrim, on passing through a 
bed of coal, reduces it to a cinder for the space of nine feet 
on each side. At Cockfield Fell, in the north of England, a 
similar change is observed. Specimens taken at the distance 
of about thirty yards from the trap are not distinguishable 
from ordinary pit-coal; those nearer the dike are like cin¬ 
ders, and have all the character of coke; while those close 
to it are converted into a substance resembling soot.J 
It is by no means uncommon to meet with the same rocks, 
even in the same districts, absolutely unchanged in the prox¬ 
imity of volcanic dikes. This great inequality in the effects 
of the igneous rocks may often arise from an original differ¬ 
ence in their temperature, and in that of the entangled gases, 
such as is ascertained to prevail in different lavas, or in the 
same lava near its so^irce and at a distance from it. The 
power also of the invaded rocks to conduct heat may vary, 
* Geol. Trans., 1st series, vol. iii., p. 210, and plate 10. 
t Ibid., vol. iii., p. 213; and Playfair, Illust. of Hutt. Theory, s. 253. 
t Sedgwick, Camb. Trans., vol. ii., p. 37. 
