284 PHOCEEDIXGS OF THK GEOLOGICAL SOCIETY. [^pr. 5, 



the passage of the great system of divergent dykes across faults of 

 every age and through the different geological formations up to and 

 including the chalk — these are facts which make it sufficiently 

 evident that in the north-western part of the British area, along the 

 great hollow stretching from Ireland northwards between the chain 

 of the Outer Hebrides and the Scottish mainland, volcanic action 

 was abundantly manifested in miocene times. Whether the erup- 

 tions took place wholly within the mioceue period, or whether they 

 extended beyond it, into later ages, remains yet uncertain. That 

 the time during which the eruptions continued was of enormous 

 duration, is shown by several considerations : — (1) The plateaux are 

 made up of many successive sheets, each of which marks at least 

 one, and sometimes more separate eruptions. In Antrim these 

 sheets rise one over another for a thickness of sometimes 900 feet ; 

 and how much thicker they may have been cannot now be deter- 

 mined, seeing that the upper part of the series has been removed by 

 denudation. In Mull there is a visible thickness of more than 

 3000 feet of volcanic beds ; yet there, too, the upward continuation 

 of them has been worn away, and there are now no means of mea- 

 suring what the original total thickness may have been. (2) There 

 occur among the basalts intercalated layers of tuff, clay, and coal, indi- 

 cating pauses between the eruptions of long enough duration for the 

 growth and accumulation of vegetable matter sufficient, when com- 

 pressed, to form two or three feet of coal*. The leaf-beds of Mull 

 likewise indicate long and tranquil intervals between the outflow of 

 successive sheets of basalt. (3) But the most striking evidence of 

 the long continuance of this volcanic period is furnished in the island 

 of Eigg, by the occurrence of ancient hollows worn by river-action 

 out of the basalt plateaux, and subsequently filled by the outpouring 

 of fresh lava. The latter bears thus the same relation to the more 

 ancient eruptions that the later coulees of Auvergne do to the old 

 denuded basaltic plateaux of that region. 



But not only have the last-erupted rocks been worn away, and all 

 evidence removed as to the time when volcanic action ceased to 

 manifest itself in our area ; denudation has since then been so con- 

 stant and so potent, that even of the whole mass of erupted matter 

 only disconnected fragments remain. Out of the great basaltic 

 tablelands long and wide valleys have been carved to a depth of 



regularly intercalated with the oolitic strata. As pointed out in a subsequent 

 page, I have now learned, however, after continued surreys of the other islands, 

 that this intercalation is deceptive, and that the basalts of Skye are only a pro- 

 longation of the miocene basalts of Mull. 



* Such beds of coal occur in Mull (see Proc. Roy. Soc. Edin. vol. vi. p. 72). 

 All the coal associated with the volcanic rocks of the Inner Hebrides is of con- 

 temporaneous, that is of miocene, date. The so-called oolitic coal of Skye I 

 have now no doubt is of this age ; and hence the intercalated strata shown to 

 occur in the volcanic series of Skye on the map of Scotland published by Sir 

 Roderick Murchison and myself, and meant to indicate the strijjs of coal and 

 associated strata, must be regarded not as oolitic, but as miocene. Their size was 

 necessarily greatly exaggerated, with the view of expressing the bedded character 

 of the igneous rocks. 



