OF THE TRAP-ROCKS OF SCOTLAND. 635 
beds. The ground of this conclusion is, that the stratum on which a bedded trap 
rests may be more or less altered, while that which covers it shows no sign of 
alteration, but reposes undisturbed on the upper surface of the trap, of which it 
sometimes contains pebbles, and to which it must therefore be posterior. 
The intrusive trap-rocks occur sometimes as apparently conformable beds, 
sometimes as amorphous masses, and frequently as vertical walls or dykes. In 
all cases they alter the contiguous surfaces of the rocks through which they pass, 
so that, even though they appear as bedded masses, their true intrusive character 
is made evident by the metamorphism which they have produced on the under 
surface of the strata which cover them. They are accordingly always subse- 
quent to the rocks with which they are associated; and though they of them- 
selves afford no test of their age, it can sometimes be approximately reached by 
an examination of the surrounding country. This classification of the trap- 
rocks is an eminently geological one, and as such depends for its extension 
to all the trappean districts of the country,—not upon the identification of the 
mineralogical similarity of the rocks, but upon a large and thorough survey of 
their geological relations. Since, therefore, the trap family plays so important 
a part in the geology of Scotland, it becomes of the highest importance to ascer- 
tain how far these rocks are referable to distinct geological periods. No geolo- 
gical map of the country can be considered complete which does not indicate in 
some way the horizons of its igneous rocks. This is a task, however, which still 
remains to be done; and the present paper, containing the results of about five 
years’ labour in various trappean districts of Scotland, must be regarded as 
merely a preliminary outline of the work that has to be performed before we 
can regard the history of igneous action in the country as in any way complete. 
I shall place here in order examples of trappean districts of successive ages, and 
show how their geology may be satisfactorily worked out, while I hope from time 
to time to be able to lay before the Society other instances, extending our know- 
ledge of the chronology of the trap-rocks of Scotland. 
Trap-rocks of Scotland of Different Ages. 
It is well-known that the trap-rocks of Scotland are of various ages,—that 
some, as those of the Sidlaw Hills, are intercalated in the Old Red Sandstone; 
that a large number are Carboniferous; that some are probably Oolitic; and 
that others are without doubt Tertiary. At the same time, it would not impro- 
bably be correct to say, that of by much the larger part of the trappean hills of 
the country little further is known than that they are trappean. Their age, 
the details of their structure, their inter-connection with each other, and their 
relations to the stratified rocks among which they occur, are subjects in which 
almost nothing has been done. SBy far the most philosophical account of any 
of the Scottish traps was that given of Arthur’s Seat and Pentland Hills by 
