
Parr VIL Sroni.§3] DYKES. BBB 
Dykes differ from veins in the greater parallelism of their sides, 
their verticality, and their greater regularity of breadth and per- 
-sistence of direction. They sometimes occur as mere plates of rock 
not more than an inch or two in thickness, at other times they attain 
a breadth of twelve fathoms or more. The smaller or thinner 
dykes can seldom be traced more than a few yards; but the larger 
examples may be followed sometimes for miles. Thus in the south 
and west of Scotland a remarkable series of basalt-dykes can be 
traced across all the geological formations of that region, including 

















































































































































Fic. 281.—Dyxers in Voutcantco Turr or A “ NEOoK,” SHore, Enis, Fire, 
the older Tertiary basalt. They run parallel to each other in a 
general north-west and south-east direction for distances of 20 and 
30 miles, and have been assigned to the great volcanic activity of 
the Miocene period. A remarkable dyke of the same series crosses 
the north of England from near the coast of Yorkshire for fully 60 
miles inland. 
Though the wall-like form is predominant among dykes, it may 
readily pass into vein-like ramifications and into intrusive sheets 
(Fig. 277). The molten material took the channels that happened 
to be most available. If the fissure bent off at an angle from its 
previous course, or if another adjacent fissure happened to be more 
convenient, the eruptive rock might change its course. Again, 
while the chief mass of ascending lava rose in one main fissure, 
portions of it might find their way into neighbouring parallel rents, 
and enclose wall-like portions of rock within the dyke, as in Fig. 282, 
where the total breadth of the main dyke, including the sandstone 
between the two arms, is about 30 feet, the sandstone being gently 
inclined, and the portion enclosed within the corner of the dyke 
having been greatly indurated. 
