STRUCTURE OF ERUPTIVE ROCKS 203 
dykes give rise to conspicuous surface-features—forming, as 
the case may be, either prominent ridges or elongated 
depressions, according as the basalt or the rock it traverses 
has offered the stouter resistance to denudation. When the 
former is the case, a dyke may rise wall-like above the 
general level of the country, continuing its course unin- 
terruptedly for a longer or shorter distance across hill and 
dale. When, on the other hand, the rocks it cuts are more 
resistant than itself, a dyke indicates its presence by a long 
narrow trench or depression instead of a prominent ridge. 
The course followed by a dyke is, as a rule, remarkably 
straight or direct, though often gently sinuous. Occasionally, 
however, this regularity may be interrupted by one or more 
zig-zags or sharp bends. It is noteworthy that dykes which 
traverse sandstones and shales are usually straighter or more 
regular than those which cut through greywackés, crystalline 
igneous rocks, and schists. While some dykes have come up 
along lines of dislocation or true faults—the great majority 
occupy fissures or rents along which no displacement has 
mecunred (Plates XLIV., XLV:). 
Dykes vary in extent—some being considerably less than 
a mile in length, while others have been followed for distances 
of 50 or even 70 miles and more—often preserving throughout 
their course a wonderfully uniform thickness. Some of the 
smaller dykes do not seem to be more than a few inches in 
thickness—the longer ones, however, are much thicker, and 
BeMmetiMies Teach, or even exceed, 100 feet’ in width, - But 
although the shorter dykes usually tend to be thin, and 
tae longer ones tobe thick, there is really no definite 
proportion between the extent and the width of dykes in 
general, A dyke 20 feet thick may have a longer range 
than one double its width—or the converse may be the case. 
Although no general average can be given for the thickness 
of the more persistent dykes, yet it may be said that dykes 
measuring 20 to 40 feet across are among the commonest 
of those which have been followed for any considerable 
distance. 
Occasionally, a dyke divides into two or more smaller 
ones—each pursuing the same general direction. Now and 
again, also, eruptive veins and veinlets proceed from a dyke, 



