124 
BASALTIC DIKES. 
low or deep red colour, and the chalk is highly 
phosphorescent. 
Fig. 35. 
111, Chalk changed into marble. 2 2, Trap dikes. 
In Anglina, a dike 134 feet wide, consisting of a 
rock composed of feldspar and augite, penetrates 
strata of shale and argillaceous limestone, which it 
cuts perpendicularly ; these are found changed to 
a distance of thirty-five feet from the edge of the 
dike. The shale, as it approaches the trap, be- 
comes gradually more compact, and is most indu- 
rated where nearest the junction. Here it becomes 
a hard porcellaneous jasper. The limestone also 
loses its earthy texture as it approaches the dike, 
and becomes granular and crystaUine. The altered 
shale also contains crystals of garnet and analcime^ 
while they are found in 7io other portion of the rock. 
In the county of Antrim, also, chalk with flints 
is traversed by basaltic dikes, and changed, for a 
distance of ten feet from the dike, into granular 
marble. 
In the same manner, red sandstone has been con- 
verted into hornstone,* and soft slate clay changed 
into flinty slate. f In Ireland, 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. The same fact has been noticed at 
Cockfield Fell, in the north of England. Here spe- 
cimens taken at a distance of thirty yards from the 
dike resemble common pit-coal ; those nearer the 
* Geol. Transactions, first series, vol. iii., p. 201. 
t Ibid., 205. 
