ANNIVERSAKY ADDRESS OF THE PRESIDENT. I05 



The geologist who traces from, older to younger formations the 

 progress of some persistent operation of nature observes the 

 evidence gradually to increase in amount and clearness as it is 

 furnished by successively later parts of the record. He finds that 

 the older rocks have generally been so dislocated and folded, and 

 are often so widely covered by younger formations, that the evidence 

 which they no doubt actually contain may be difficult to decipher 

 or may be altogether concealed from view. In following, for 

 instance, the progress of volcanic action, as he passes from the 

 older to the younger Palaeozoic chronicles, he is impressed by the 

 striking contrast between the fulness and legibility of the Carboni- 

 ferous records and the comparative meagreness and obscurity of 

 those of the earlier periods. The Carboniferous rocks have under- 

 gone far less disturbance than the Silurian and Cambrian forma- 

 tions ; while over wide tracts, where their volcanic chapters are 

 fullest and most interesting, they form the rocks at the surface, and 

 can thus be subjected to the closest scrutiny. Hence the remains 

 of the later volcanic phenomena of the Palseozoic periods present a 

 curiously modern aspect when contrasted with the fragmentary and 

 antique look of those of older date. 



Two great phases or types of volcanic action in Carboniferous 

 time may be clearly distinguished: — 1st. Plateaux, where the 

 volcanic materials were discharged so copiously that they now form 

 broad tablelands or ranges of hills, sometimes many hundreds of 

 square miles in extent; 2nd. Purs, where the ejections were often 

 confined to the discharge of a small amount of fragmentary 

 materials from a single independent vent, and where, when lavas 

 and more copious showers of ash were thrown out, they generally 

 covered only a small area round the volcano which discharged them. 

 I shall trace the distribution and characters of each of these types. 



(A) The Plateaux. 

 Under this term I group all the more copious eruptions during 

 the Carboniferous period, when the fragmentary materials generally 

 formed but a small part of the discharges, but when the lavas were 

 poured out over so wide an area as to form lava-fields sometimes 

 more than 2000 square miles in area, and so persistently as to 

 build up a pile of volcanic material sometimes upwards of 1500 feet 

 in thickness. This phase of volcanic action was especially charac- 

 teristic of the earlier part of the Carboniferous period across the South 

 of Scotland, but does not appear elsewhere in the same system in 



