489 
of Edinburgh , Session 1868 - 69 . 
in Paris in the year 1822 as an “ Essai Gfeologique sur l’Ecosse.” 
This volume, besides its detailed descriptions of the rocks of the 
country, contains a coloured geological map, and a series of plates 
of illustrative sections. The map has a peculiar interest in our 
present inquiry. The outlines of the geological structure of the 
kingdom are there broadly sketched. He traces the general area 
occupied by the gneiss, and what he calls the “ chloritic and 
quartzose rocks” of the Highlands, the position and extent of the 
great coal-field of the Midland Valley, and the limits of the 
“greywacke” of the Southern Uplands. He distinguishes the 
granites, syenites, and porphyries, and roughly separates the rocks 
of volcanic or trappean origin, according to their respective ages. 
In this latter subject his observations were far in advance of his 
time. He had studied some of the extinct volcanic districts of the 
Continent, and gained a familiarity with volcanic phenomena which 
gave him a great advantage over his Scottish contemporaries, who 
were still disputing whether or not the trap-rocks of their country 
had really a volcanic origin. The map on which Dr Boue inserted 
his broad generalisations was of course merely an outline or sketch, 
necessarily full of inaccuracies and omissions. But in some im- 
portant respects it foreshadowed the maps that have succeeded it.* 
While the Edinburgh School of G-eologists continued to furnish 
descriptions, and sometimes also maps of different geological dis- 
tricts, Dr MacCulloch carried on his own independent researches, 
and brought them at intervals before the Geological Society of 
London. He had been required by the Board of Ordnance to make 
some special investigations, partly with a view to discover some one 
mountain, the geological structure of which might afford a prospect 
of repeating with greater reliability the experiments made by Dr 
Maskelyne upon Schihallien to estimate the weight of the earth. 
In the course of these and other journeys he had noted upon a 
map the geological structure of each district examined ; and find- 
ing at last that a considerable area of the country had been thus 
delineated, he represented to the Board of Oidnance the propriety 
* Dr Boue still retains his youthful enthusiasm. He is resident at Vienna, 
from which place I have recently had some interesting letters from him, full 
of gossip about Edinburgh in the early part of this century, and of references 
to his own wanderings in Scotland. 
