74 Proceedings of the Royal Society 
and there now seems to be a strong probability that it will turn 
out to be of Miocene age. Parts of that syenite are true granite, 
while the lias around it has suffered an extensive metamorphism. 
It will be an important addition to our knowledge of the history of 
metamorphic action, if the alteration of the secondary rocks of the 
Hebrides is eventually shown to be connected with the evolution 
of volcanic rocks during the Miocene period. 
The wide extent to which the British Islands were affected by 
the Miocene volcanoes of the west was then referred to. That ex- 
tent is not to be measured by the area at present covered with 
tertiary volcanic rocks, nor even by the area which these rocks 
may have originally overspread, but from which subsequent denuda- 
tion has removed them. From the great volcanic ridge running 
through Antrim and the Western Islands, thousands of trap- 
dykes diverge in a south-easterly direction. They become fewer 
as the distance from that bank increases, yet they extend as 
far as the coast of Yorkshire. Ho single dyke, indeed, has been 
traced across the country from sea to sea, but there can be little 
doubt that they all belong to one series. They cut through 
all the formations up to and including the chalk, and they like- 
wise traverse the older portions of the tertiary volcanic rocks. 
They must thus be of tertiary age, and belong to that series of 
igneous masses described in the present paper. They do not 
usually run along lines of fault ; on the contrary, they are found 
to cross faults of fifty fathoms and upwards without being deflected. 
Their evenness and parallelism show that they must have ascended 
through fissures prepared for them by subterranean movements. 
Thus we learn that in tertiary times the greater part of Scotland, 
the north of England, and the north of Ireland, were cracked 
by earthquakes, and that liquid lava rose through the hundreds 
of parallel rents, perhaps in some cases actually reaching the 
surface. 
The last section of the paper was devoted to an account of the 
denudation of the tertiary volcanic rocks. It was shown that wide, 
deep, and long valleys have been excavated out of the horizontal 
trap-beds ; that these rocks have sometimes been so wasted away 
that only huge detached pyramids of them are left, as in the case 
of Ben More, Mull ; that the volcanic bank has been worn down 
