zmth those of a similar Nature found in Scotland. S31 
we cannot identify them with the true trachytes. But this is 
a point which must be determined by you. 
I have only further to remark, in regard to the trachyte, that 
some of the varieties found in masses in volcanic tuffa, brought 
to my recollection certain porphyries of the Braid Hills near 
Edinburgh. This, however, may possibly prove but an acci- 
dental 'resemblance ; and this is the more likely as the Braid 
Hills, Blackford Hill and Pentland Hills, which you point out 
as members of the same series, differ in many things from true 
trachyte districts. 
8. I did not observe any where in the volcanic districts of 
Auvergne and the Vivarais, rocks like the greenstone forming 
the great vein in the coal-field to the N. E. of the Calton Hill, 
nor any which corresponded with the following, viz. the green- 
stone of Corstofphine Hill and of Cramond ; the greenstones 
of Burntisland in Fifeshire; the greenstone of the Pentlands; 
the clinkstone-porphyries of Dunbar ; the Invergourie rocks 
near Dundee ; the trap-rocks of Renfrewshire. Yet still our 
knowledge of volcanic-rocks is so imperfect, that we cannot 
maintain that all these, and others of the same description, are 
of Neptunian origin. 
9. Nowhere in the volcanic districts, already so often men- 
tioned, did I find any amygdaloidal rocks resembling those of 
the Pentlands, Montrose, and other parts of Scotland. The 
only rock which made an approach to these, was that of the 
Puy Giroud near Clermont. 
In conformity with the preceding, and indeed with other ob- 
servations to be communicated to you, I would propose arran- 
ging your trap-rocks in the following manner, according to their 
probable origin. 
* Evidently Volcanic. 
1. Calder basalt, and the one resembling it which I found 
i;iear Edinburgh. ^ 
% The Giant’s Causeway. 
Probably Volcanic. 
1. Staffa. % Some parts of the Island of Mull. 3. Arthur 
Seat, in part. 4. Dalmahoy Hills. 5. Lamlash Isle. 6. The 
trap-veins in Arran, and in the neighbouring districts in Ireland 
Y 2 
