494 W. J. SOLLAS ON THE SILURIAN DISTRICT 
fairly conclude that the conditions which did overspread the whole 
area were those of the Welsh Old Red. 
According to the view just advanced the Old Red Sandstone is 
regarded as the complete equivalent of the Devonian strata, the dif- 
ference between the Welsh and Devonshire areas chiefly being that 
in the latter we have an interdigitation of rocks (fossiliferous beds) 
formed under Deyonian conditions with other rocks formed under 
Welsh conditions (whatever those may have been), while in the 
former the beds of Old Red Sandstone were deposited under condi- 
tions which remained approximately the same from the beginning 
to the end of the period. 
The Old Red Sandstone of South Wales is a continuous deposit 
from the conformable Silurian at its base to the conformable Car- 
boniferous at its summit; and hence nothing can be clearer than the 
inference that whatever formations or unconformities occur else- 
where between the top of the Silurian and the bottom of the Car- 
boniferous, they must one and all have taken place during the time 
that the Welsh Old Red Sandstone was in process of formation ; and 
whatever correlation may be made of the Devonshire beds with 
corresponding rocks in Ireland or on the Continent, they must at 
all events, so far as they are admitted to lie between the Silurian 
and Carboniferous, be regarded as having been formed during the 
whole or a part of the time known as the Old Red Sandstone period ; 
the genuine Devonian may be the equivalents of the whole or a part 
of the Old Red Sandstone, but no more. 
A word finally, if it be not too far from the point, as to the dis- 
tribution of areas of elevation, or minimum depression, and of 
maximum depression over the West-of-England Old Red Sandstone 
area. The evidence, so far as it goes, certainly points to the exis- 
tence between the meridians of 3° and 4° of iongitude of three more 
or less east-to-west areas of minimum depression, and of two inter- 
vening areas of maximum depression. The zones of minimum 
depression were (1) a northernmost area over the north part of 
Wales, which, as shown by the absence of Old Red and the uncon- 
formable presence of Carboniferous or Silurian rocks, remained dry 
land during the Old Red period ; (2) a southernmost area, probably 
the northward extension of the land which afterwards supplied 
Carboniferous sediments to Devonshire, and indicated partly by the 
Cambrian rocks of the Dodman and Start Point, which almost cer- 
tainly formed land during the Silurian period ; and (3) the barrier 
along the Bristol Channel and south of the Mendips, suggested by 
Mr. Etheridge, and supported by the observations in this paper. 
The zones of maximum depression were (1) one on the north over 
South Wales and Hereford, withits axis about the northern escarp- 
ment of the South-Wales coal-field ; and (2) another on the south 
over Devonshire and Cornwall, with its axis passing from east to 
west, probably in the neighbourhood of Plymouth. 
The central barrier was subject to movements of depression of 
very varying rapidity, which led to the introduction at intervals of 
Old-Red-Sandstone conditions over the Devonshire area, and probably 
