204 STRUCTURAL AND FIELD GEOLOGY 
but this is apparently exceptional. Dykes often wedge out 
suddenly, both in lateral and vertical directions. Traced 
across country, they not infrequently seem to die out, and then 
after a shorter or longer interval they may as suddenly 
reappear. When a dyke of this kind is represented upon a 
map, therefore, we have the appearance of two or more 
dykes following each other along the same line. That the 
apparently separate dykes, however, are really portions of 
one and the same intrusion, has now and again been 
demonstrated in the coal-bearing districts—where a dyke 
has been followed continuously throughout all the coal- 
workings, although it fails in some places to reach the 
surface. Sometimes, indeed, a dyke cuts the lower coals but 
does not penetrate the higher seams in one and the same 
coal-pit. 
Basalt-dykes are jointed most prominently at right angles 
to their direction—the jointing being frequently prismatic 
(see Figs. 73, 74). But, in some cases, the joints rum parallel 



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FIG, 73.—PRISMATIC JOINTING FIG. 74.—COMPLEX PRISMATIC 
IN A DYKE. JOINTING IN A DYKE. 
to the walls, so as to give the rock a kind of flaggy struc- 
ture (see Plate XLVI.). Parallel jointing of this nature 
is usually, however, confined to the marginal areas of the 
rock, 
The rock of a dyke is almost invariably finer grained 
along its margins than towards the centre—a structure which 
is most conspicuous in the case of the thicker dykes. Thin 
dykes are usually fine-grained throughout, yet even these 
tend to be most compact towards the sides. This structure 
is obviously due to the chilling effect of the contiguous 
