Io6 PKOCEEDINGS OF THE GEOLOGICAL SOCIETr. 



Sritain. On the whole, it preceded the type of the Pays, although 

 in the centre of the Midland Yalley of Scotland the two types 

 appear to have overlapped or to have been for a short interval 

 contemporaneous. The eruptions of the Plateaux extended from 

 the close of the Old Eed Sandstone period through that section of 

 Carboniferous time which was marked by the deposition of the Cal- 

 ciferous Sandstones/ but entirely ceased before the accumulation 

 of the Main or Hurlet Limestone, which forms the base of the ' Car- 

 boniferous Limestone Series' of Scotland. But the limits of their 

 duration were not everywhere the same. Thus they seem to have 

 begun and to have ceased rather earlier in the eastern part of the 

 region. There their lavas lie directly upon the Upper Old lied 

 Sandstone containing scales of Boihriolepis and other characteristic 

 fishes, and are covered by the Cement-stone Group of the Calciferous 

 Sandstones. In the west, on the other hand, a considerable thick- 

 ness of Carboniferous strata underlies the volcanic sheets, which 

 thence extend upwards to the base of the Main (Hurlet) Lime- 

 stone. On the other hand, the type of the Puys, although it 

 appeared in Fife and Midlothian during the time of the Calciferous 

 Sandstones, attained its chief development during that of the 

 Carboniferous Limestone, and did not finally die out until the 

 beginning of the deposition of the Coal Measures. 



I. Distribution. — The geographical positions of the Plateaux are 

 easily described, for notwithstanding the eff'ects of many powerful 

 faults and extensive denudation, the general position of these 

 tracts and their independence of each other can still be traced. 

 They are entirely confined, as I have said, to the southern half of 

 Scotland. In noting their situations we are again brought face to 

 face with the remarkable fact, so strikingly manifested in the 

 geological history of Britain, that volcanic action has been apt to 

 recur again and again in or near to the same areas. The Carboni- 

 ferous volcanic plateaux were poured out from vents, some of which 

 not impossibly rose among those of the Lower Old Eed Sandstone. 



^ For the sake of clearness I use here the subdivisions of the Carboniferous 

 system which are characteristic in Scotland. The Calciferous Sandstones are 

 the stratigraphical equivalents of the lower portion of the Cai'boniferous Lime- 

 stone and Limestone Shale of England. They consist of two groups : — the 

 lower (Red Sandstone Group), formed mainly of red sandstones ; the upper 

 (Cement-stone GTroup), made up of white and grey sandstones, black, blue, or 

 green shales, bands of cement-stone, ironstone, limestone, and sometimes oil- 

 shales and thin coals. 



