216 Proceedings of Royal Society of Edinburgh. [sess. 
extremities, i.e., to the ends and the upper side of the intrusive 
mass. Wedge-shaped intrusions are much less common in the 
case of the dykes composed of basic, or of sub-basic, materials 
than in those which contain potash felspars. Why this is the 
case is not clear. 
Occasionally basic dykes are clearly seen to terminate dozen- 
wards. Sir Archibald Geikie has lately figured some examples 
from Fife which are seen to do this. But all those which do so 
belong, I think, to a different category from the one which is here 
specially under consideration, and they will he considered in that 
connection in another paper. 
It seems to he generally assumed that dykes often coincide 
with lines of fault. In the course of an extensive field experience 
I have but rarely met with cases in which it was quite clear that 
this was so : hut as geologists of good repute say that such cases 
are of common occurrence, I will not press my own convictions too 
far. It seems to me that in many cases where a dyke has risen in 
contiguity to a fault of older date that the dyke is not in the least 
influenced by the old plane of weakness. Quite commonly, how- 
ever, older dykes may deflect the course of a newer one which has 
cut obliquely across them, in a manner analogous to that which 
happens where a newer fault is “ trailed’’ by an older one — a 
phenomenon quite different in its nature from the “ heave ” pro- 
duced when an older fault and its enclosing rock are bodily shifted 
by a later thrust. This is only referred to here because there 
seems to have been some misunderstanding regarding the relative 
ages of two dykes of which one has gone off on one side of another 
dyke in a different plane from that at which the two met on the 
other. I have previously discussed this matter at some length in 
a paper on “Faults” in the Trans. Edin. Geol. Soc. for 1889, 
pp. 71-74. 
There is a fine example of the influence of an older sill upon the 
upward course of a dyke on the west shore of Carsaig Bay in Mull. 
The dyke rises through Lias Shales, and on coming near to the 
base of the sill the dyke suddenly spreads out laterally, so as to 
pass on both sides into a sill, which it does, however, without 
coalescing with the older one, or even quite reaching it. On 
either side the lateral extension of the dyke thins out within a 
