514 
ELEMENTS OE GEOLOGY. 
stone, which I observed cutting through sandstone on the 
beach near Kildonan Castle, in Arran. The larger branch 
varies from five to seven feet in width, which will afford a 
scale of measurement for the whole. 
Fig. 593. 
Ground-plan of greenstone dikes traversing sandstone. Arran. 
In the Hebrides and other countries, the same masses of 
trap which occupy the surface of the country far and wide, 
concealing the subjacent stratified rocks, are seen also in the 
sea-cliffs, prolonged downward in veins or dikes, which prob¬ 
ably unite with other masses of igneous rock at a greater 
dep'th. The largest of the dikes represented in the annexed 
diagram (Fig. 594), and which are seen in part of the coast 
of Skye, is no less than 100 feet in width. 
Fia\ 594. 
Trap dividing and covering sandstone near Suishnish, in Skye,. (MacCulloch.) 
Every variety of trap-rock is sometimes found in dikes, 
as basalt, greenstone, feldspar-porphyry, and trachyte. The 
amygdaloidal traps also occur, though more rarely, and even 
tuff and breccia, for the materials of these last may be wash¬ 
ed down into open fissures at the bottom of the sea, or dur¬ 
ing eruption on the land may be showered into them from 
the air. Some dikes of trap may be followed for leagues 
uninterruptedly in nearly a straight direction, as in the 
north of England, showing that the fissures which they fill 
must have been of extraordinary length. 
Rocks altered by Volcanic Dikes. —After these remarks on 
the form and composition of dikes themselves, I shall de¬ 
scribe the alterations which they sometimes produce in the 
rocks in contact with them. The changes are usually such 
as the heat of melted matter and of the entangled steam 
and gases might be expected to cause. 
Plas-Nerwydd: Dike cutting through Shale. —A striking ex- 
