650 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
Trap-rocks of Tertiary Age. 
Among the Trappean islands of the Hebrides and the adjacent shores of the 
mainland, there is still a wide field for investigation. My own researches have 
reached upward from the base of the Lias to the top of the Middle Oolite; and it 
seems highly probable that some connecting links will yet be found between the 
Oolites and the leaf-beds discovered by the Duke of Argyle at Ardtun in Mull. 
Meantime, however, the links are wanting: from the Middle Oolite basalts of Skye 
we pass to the Tertiary basalts of Mull. The age of the latter is marked by the 
dicotyledonous character of the leaves, which occur in bands of clay intercalated 
among beds of basalt and volcanic ashes. The existence of these leaf-beds, with 
their associated volcanic rocks, is of great interest when regarded in connection 
with the basalts of Ireland on the one side, and with the great extent of similar 
beds among the Hebrides on the other. It is not impossible, however, that traces 
of volcanic rocks of the same, that is, of tertiary age, may be found on the main- 
land, even in localities considerably removed from the Ardtun district. 
In examining the geology of Arthur’s Seat for the Geological Survey, I was 
particularly struck with the features of the newer which rest on the older volcanic 
rocks of the hill. When studied in connection with the surrounding country, the 
interval of time represented by the unconformity seemed constantly to widen, 
until it became evident that the upper series of basalts and ashes was not only 
later than the Carboniferous system, but possibly later than even any part of 
the Palzeozoic rocks. I was afterwards made aware of a suggestion of the late 
Professor Epwarp Forbes, that the upper part of Arthur’s Seat might possibly 
be of Tertiary age. Until a large portion of Scotland has been minutely 
examined, it will be impossible to decide this question. In the meantime it is 
sufficiently evident that these Arthur’s Seat rocks are long posterior to the Car- 
boniferous system, and if not tertiary, must at least belong to the later part of 
the Secondary series.* 
Later Trap-dykes of Scotland. 
It is a point of some interest to determine the general direction and probable 
age of the great greenstone or basalt dykes, by which all the rocks of Scotland, 
whether of aqueous or igneous origin, are more or less traversed. Their direc- 
tion, when viewed in a broad way, is strikingly persistent from north-west to 
south-east—7.¢., across the prevalent strike of the rock-formations. Variations 
occur in many localities, being determined either by some change in the strike of 
the rocks, or more usually by the position of faults along which the igneous 
matter has risen from below. 
* For a full statement of the evidence on which this conclusion is: based, see “ Geology of Edin- 
burgh” (Mem. Geol. Surv.). 

—— 
