Royal Society of Edinburgh. 143 
century past. And Loudon, in his most laborious and valuable 
“‘ Arboretum et Fruticetum Britannicum,” arranges and digests much 
of these materials. One important use served by the authors named 
is to enable us to compare the present condition and size of trees 
with what they were at ascertained distances of time previous; while 
the collection of returns of remarkable trees now making to the 
Highland Society, will serve as a foundation for such measurement 
and comparison in future times. 
Monday, 15th April 1861.—The Hon. LORD NEAVES, 
Vice-President, in the Chair. 
The following Communications were read :— 
1. Additional Observations on the Chronology of the is 
Rocks of Scotland. By Archibald Geikie, Esq., F.G.S 
In a communication made to the Society last session, the author 
stated the results of a series of explorations among the trap-rocks 
of Scotland, and showed that, at successive periods, during the depo- 
sition of the Lower Silurian, Old Red Sandstone, Carboniferous, 
Oolitic, and Tertiary formations, there were contemporaneous erup- 
tions of volcanic material. During the year 1860, the investigation 
was continued across the Highlands into the Inner Hebrides, and 
throughout a large part of the central counties southward to the 
Cheviot Hills. The author was now able to fill in more fully what 
had only been sketched in outline in the previous paper, and to pre- 
pare a series of maps to illustrate the voleanic areas of Scotland 
during the successive geological periods. He showed that, in the 
Scottish Highlands, no distinct trace existed of any igneous rock 
erupted contemporaneously with the deposition of those Lower 
Silurian strata which are now metamorphosed into gneiss, mica- 
schist, clay-slate, &c. The greater part of his observations during 
the past year had been devoted to the elucidation of the chronology 
of the igneous rocks belonging to the period of the Old Red Sand- 
stone, and he found that, in central Scotland, that formation exhi- 
bited a copious series of contemporaneous felstones and ash-beds in 
its lower and upper members; the former being exemplified in For- 
farshire and Perthshire, and the latter in Fife and in the Pentland 
Hills. Several additional facts had also been observed among the 
Carboniferous trap-rocks, tending to make the series more complete, 
and to show how with volcanic movements there were associated 
certain risings and sinkings of the land, whereby the fauna and flora 
of the Carboniferous period were locally modified. Reference was 
