636 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
Mr MacLaren upwards of twenty years ago,* and his work will ever rank among 
the classics of British geology. Much, undoubtedly, has been done in the mine- 
ralogy of the trap-rocks, as the works of JamEson, Boun, MaccuLuocn, and others 
sufficiently show; but in the department of trappean geology the labourers 
have been comparatively few. A wide and well-nigh untrodden field of research 
thus lies open, than which no other branch of physical geology in Scotland will 
yield a richer harvest of results. 
Traces of Trappean Rocks among Lower Silurian Strata. 
The metamorphic rocks which constitute the Scottish Highlands, though 
abounding in granites, syenites, porphyries, serpentines, and other so-called 
igneous rocks, do not appear to show any evidence of truly interbedded trap- 
rocks. If any such originally existed, they must have suffered from the meta- 
morphic action which has produced the gneiss and schists of the Highlands, 
and which would undoubtedly impart to any lava-form rock a new and very 
different lithological character. It seems vain, therefore, to hope for the dis- 
covery of igneous rocks ejected over the area of the Highlands during the deposi- 
tion of the strata which are now visible as gneiss and schist. 
It is otherwise, however, with the equivalents of these strata in the south of 
Scotland. There the greywacke and slate are comparatively little altered ; and we 
may reasonably hope yet to detect amongst them traces of contemporaneous vol- 
canic action. Throughout the lower Silurian grits and conglomerate bands of the 
Lammermuir Hills, felspathic material is especially abundant. I do not refer to 
the dykes and masses of felstone by which these strata are everywhere traversed, 
but to the material of which the strata are themselves composed, and which, of 
course, points to the existence of rocks at or near the sea-level, when the grits 
and shales of the Lammermuirs were deposited. Many of the grits consist of a 
yellow felspathic paste, with small rounded grains of quartz. Such beds scarcely 
differ from certain Old Red grits among the Pentland Hills, which lie upon and 
have been formed out of yellow felstones. It seems probable, therefore, that some 
of the currents which deposited sediment across the site of the Lammermuirs 
came from a region where felspathic trap abounded, similar, apparently, to part 
of what at the present day forms the chain of the Pentlands. Pebbles of various 
felstones are likewise of frequent occurrence among the thin conglomerate bands 
intercalated in the Lammermuir grits. It may be mentioned, in passing, that 
these features are not confined to the low place in the Silurian series represented 
by the grits of Berwickshire. I have traced the same admixture of felspathic 
matter in the grits and conglomerate bands throughout the entire thickness of 
the Lammermuir series, and across into the grits of the Pentland Hills, which an 
* Geology of Fife and the Lothians. Edinburgh, 1839. 


