324 ON THE ANCIENT GRAUWACKE 
rian fossils are said to occur,’ —a circumstance not unfre- 
quent,” it is added, “in the Mountain Limestone of Scotland.” 
No one, however, is now more thoroughly convinced than Pro- 
fessor Nicol, that the Silurian organisms of Girvan are not 
organisms of the Carboniferous series; that, on the contrary, 
they definitely determine the place and age of the deposits in 
which they cecur as Lower Silurian; and further, that they 
throw more light on the history of this ancient system, in its 
development in the southern Highlands, than the fossils of all 
our other Scottish localities put together. 
In January 1848, Mr. Nicol, at that time Assistant. Secre- 
tary of the London Geological Society, read before that body 
a paper on the Silurian Rocks of the Valley of the Tweed, 
which was afterwards published in the Journal of the Society. 
Even at a period so recent he could properly state, in his intro- 
duction, “that there is perhaps no extensive formation in the 
British islands of which we possess less certain geological 
knowledge than of the rocks constituting the great mountain 
chain which crosses the southern counties of Scotland from 
east to west.” His paper, however, served to add considerably 
to the little previously known regarding the deposit. Among 
the fossils by which it was illustrated, Mr. Salter recognized 
the fragments of five genera of trilobites, and an equal number 
of genera of shells, chiefly brachipods, all of a character indi- 
cative of the Lower Silurian group. About the same time 
a collection made from the Grauwackes of the shores of Kirk- 
cudbright was submitted to the London Geological Society by 
Lord Selkirk, and was found to be of an Upper Silurian char- 
acter; indeed, as appeared from the identity of some of the 
fossils, of the age of the Wenlock shale. In the May of the 
same year in which Professor Nicol submitted his paper to the 
public, the subject was still further elucidated in a valuable 
memoir, by Mr. Carrick Moore, Secretary to the Geological 
