206 Proceedings of Royal Society of Edinburgh. [sess. 
section at Farragandoo Cliff, at the west end of Fair Head, which 
shows this indigitation of dolerite with the country rock in a 
manner which is thoroughly typical of the behaviour of sills in 
that respect. It will he observed that there is little evidence, if 
any, of mechanical disturbance. On the contrary the whole mass 
of field evidence seems to point to the intrusive rock having taken 
the place of the shale, without causing any uplift of the rock 
surfaces which are supposed by some writers to have been thus 
laccolitised. The relationship of the one rock to the other is 
certainly not of the kind that might he illustrated by thrusting 
one’s fingers between the leaves of an otherwise closed hook lying 
upon its side. The separation of the leaves forming the upper 
half of the hook from these forming the other would in such a 
case hear an exact proportion to the size of the fingers thrust in ; 
and there must in all such cases he a certain amount of curvature 
of the upper part, which occasions some lateral movement of the 
•ends of the separated parts relative to their position before the 
“ intrusion.” This shortening, supposing the uplift to take place on 
one side only, would be proportionate at either end of the uplifted 
part to half the difference between the length of the arc formed by 
the lifted portion and half the length of the portion undisturbed. 
In a rock thus acted upon the adjustment to the changed lateral 
dimensions must occasion some mechanical disturbance. Traces of 
such I have never met with. In actual examples the case is rather of 
that kind which might happen if part of the leaves, corresponding 
in shape and in volume to those of the fingers thrust in, had been 
cut out. In the former case, supposing we are dealing with a 
•closed book lying on its side, the outer cover would be lifted ; if 
the case were of the latter kind, the cover might remain quite 
undisturbed while the fingers were pushed in. I shall adduce some 
further evidence in support of the view that the case last 
illustrated is the usual one ; though there may well he some 
occasional exceptions to it. Fig. 9, on page 207, shows a well- 
known case of intrusion at Dodhead Quarry, near Burntisland Golf 
Course. Fig. 10, from the same quarry, is traced from a photograph 
taken by Professor Reynolds of Bristol, in which yet another 
example occurs of an intrusive rock eating its way into shales, which 
remain undisturbed above and below. Its position is shown by a B 
