Qoressiiy 
XXIX.—On the Chronology of the Trap-Rocks of Scotland. By ARCHIBALD 
GEIKIE, Esq., F.G.S. (One Plate, XX XVIII.) 
(Read 19th March 1860, and 15th April 1861.) 
In the Geology of Scotland, no feature comes at once more prominently before 
the observer than the number and variety of the trap-rocks.* From the Sheant 
Isles to the Cheviot Hills the map is dotted over with trappean patches, which 
occur sometimes as long, narrow dykes, and often as irregular sheets, that extend 
almost over entire counties. The appearance of the rocks on the face of the 
country is scarcely less marked than on the map, and to their ever-changing 
varieties, we owe not a little of the characteristic scenery of Scotland. Through- 
out the wilds of the Inner Hebrides it is the trap-rocks which form many of the 
precipitous cliff-lines, and the craggy irregular hills. The soft pastoral valleys of 
the Ochils and the Pentlands lie among trappean rocks, while the hill-ranges, 
which break up the great central valley of Scotland, more especially the abrupt 
solitary crags that form such prominent landmarks, owe their existence to the 
permanence of the trap-rocks of which they consist. In short, there is no group: 
of rocks more constantly found throughout the length and breadth of the island, 
and none, therefore, which so frequently obtrudes on the geologist or more im- 
peratively demands his attention. 
Definition and Classification of Trap-Rocks. 
Under the designation of trap are included, in this communication, all the 
igneous rocks of the country which do not fall within the limits of the granite 
family ; excluding, however, at least in the meantime, such rocks as the hypers- 
thene of Skye, which I have long suspected to be metamorphic. The usual classi- 
fication of trap-rocks is according to their mineralogical characters. But in 
unravelling the structure of a trappean region, such a classification will be found 
in many cases insufficient. It is the great province of the geologist to determine 
the succession in time of the materials with which he deals. His terms are, for 
the most part, either founded on this chronological order, or are virtually referred 
to it, whatever be their real import. By Oolite, for example, he means not neces- 
sarily a roe-like stratum, but a series of strata, whether roe-like or not, possessing a 
* The word trap is here used in a generic, not in a specific sense. It includes greenstones, basalts, 
felstones, ashes, &c.; in short, all the truly igneous rocks of former geological periods, whether volcanic 
(erupted at the surface) or intruded as dykes and irregular masses into other rocks. Granite, ser- 
pentine, and the granitoid porphyries I regard as not truly igneous, and they are therefore excluded. 
VOL. XXII. PART III. 8B 
