ANNIVEESAEY ADDEESS OF THE PEESIBENT. I09 



been some intermediate ground that escaped being flooded with 

 lava from the vents of the Merse, on the one hand, and of the 

 Solway on the other. The Solway lavas form a much thinner 

 group than those of any of the other plateaux. Prom the wild 

 moorland between the sources of the Liddell and the E-ule Water, 

 they run in a narrow and much-faulted band south-westward across 

 Eskdale and the foot of Annandale, and are traceable in occasional 

 patches on the farther side of the Nith along the southern flanks of 

 Criffel, even as far as Torrorie on the coast of Kirkcudbright — a 

 total distance of about 45 miles. It is probable that this long out- 

 crop presents merely the northern edge of a volcanic platform 

 which is mainly buried under the Carboniferous rocks of the Solway 

 basin. Yet it exhibits all the chief characters of the other plateaux, 

 and even occasionally rivals them in the dignity of the escarpments 

 which mark its progress through the lonely uplands between the 

 head of Liddesdale and the Ewes Water. Together also with the 

 Merse plateau it illustrates in a striking manner the distribution 

 of the volcanic eruptions along the valleys and low plains. The 

 vents from which the lavas and tuflPs proceeded are chieflj^ to be 

 found on the lower grounds, though these bedded volcanic rocks rise 

 to a height of 1712 feet (the Pikes) to the west of the Cheviot Hills. 

 Between the Silurian uplands of Selkirkshire and Berwickshire on 

 the north and the ridge of the Cheviot Hills on the south, the 

 broad plain was dotted with volcanic vents and flooded with lava, 

 while to the south-west the corresponding hollow between the 

 uplands of Dumfries and Galloway, on the one side, and those of 

 Cumberland on the other, was similarly overspread. The signifi- 

 cance of these facts will be more apparent when the grouping of 

 the vents has been described. We shall then also be better able to 

 realize the validity of the inference that the present plateaux are 

 mere fragments of what they originally were, wide areas having 

 been removed from the one side of them by denudation, and having 

 been concealed on the other under later portions of the Carboniferous 

 system. 



II. Peteogeaphy. — The volcanic materials characteristic of the 

 plateau-type of eruptions consist mainly of lavas in successive 

 sheets, but include also various tufl's in frequent thin courses and 

 less commonly in thick local accumulations. The lavas are chiefly 

 porphyrites, but these vary a good deal in the relative proportions 

 of silica. Some of them become markedly basic and pass into 

 dolerites and olivine-basalts. With these rocks are occasionally 



