126 PEOCEEDI^fGS OP THE GEOLOGICAL SOCIETY. 



Hill overlie the red sandstone group and pass underneath the upper 

 group of white sandstones and black shales — a platform whereon 

 the plateau rocks should have occurred had they extended over that 

 district. There appears indeed to he in this locality a commingling 

 of the petrographical types both of the Plateaux and of the Puys, 

 for both in Arthur Seat and Calton Hill the higher lavas are por- 

 phyrites not unlike some of those in the plateaux. Over the 

 western part of Midlothian, the eastern portion of West Lothian, 

 and the southern margin of Pife abundant traces occur of puy- 

 eruptions during the deposition of the upper group of the Calciferous 

 Sandstones. Elsewhere in Central Scotland there is no evidence of 

 the vents having been opened until after the deposition of the Main 

 Limestone. They remained active in West Lothian until near the 

 close of the time represented by the Carboniferous Limestone ; but 

 in Ayrshire they continued in eruption until the beginning of the 

 accumulation of the Coal Measures. These western examples of 

 the puy-type are the latest known, as the eastern instances round 

 Edinburgh are the earliest. 



I. Distribution. — In tracing the geographical distribution of the 

 puy-eruptions we are first impressed with the force of the evidence 

 for their extremely local and restricted character. Thus in the area 

 of the basin of the Eirth of Eorth traces of them are abundant to 

 the west of the line of the Pentland Hills, while to the east of that 

 line not a vestige can be detected, though the same series of strati- 

 graphical horizons is well developed on both sides of the Lothian 

 coal-field. Again, to the westward of this central district of puys, 

 over the area of Stirlingshire, Lanarkshire, and Renfrewshire lying 

 to the north of the great Ayrshire plateau, no record of puy- 

 eruptions has been noticed. Immediately to the south of that 

 plateau, however, these eruptions were numerous in the north of 

 Ayrshire. 



Another fact which at once attracts notice in Scotland is the way 

 in which the puy-vents have generally avoided the areas of the 

 plateaux, though they sometimes approach them closely. As a rule, 

 it is possible to distinguish the tuffs and agglomerates which have 

 filled up these vents from those which mark the sites of the eruptive 

 orifices of the plateaux. There are, no doubt, some instances, as in 

 Liddesdale, where puys have appeared on the sites of the older 

 lavas, but these are exceptional collocations.^ On the other hand, 



^ A means of definitely placing some of these vents in the series of puy- 

 eruptions is stated farther on, at p. 145. 



