488 Scientific Proceedings, Royal Dublin Society. 
but no one deposit is universal, each extends only over a certain 
area, sometimes an unexpectedly limited one. 
If, in the Atlantic Ocean basin, we followed the mode of calceu- 
lation now being deprecated, we should arrive at vast thicknesses 
of strata that cannot possibly be correct. The different deposits 
now being laid down in the Atlantic, although often strikingly 
different, and sometimes even definitely limited, are contempo- 
raneous accumulations, whether they adjoin each other or are 
separated by thousands of miles, and that, though one may be 
shingle, another gravel, or sand, or clay, or limestone, or peat, 
and also though some are very thick and others extremely thin. 
Yet, if these strata were to be elevated into land, it is probable 
we should be asked to believe in numerous intervals of interrupted 
deposition, because the accumulations of different materials are 
not all found everywhere present. 
In illustration of what has been said I may refer to the Silurian 
rocks of the United Kingdom, dwelling more particularly on 
those in Ireland with which I am best acquainted. Under the 
term SituRIAN, I include the rocks usually called “ Upper Silu- 
rians” and “ Lower Old Red Sandstone.” 
In former times geologists called certain rocks “Red Sandstone,” 
solely from their colour and lithological character, but it was 
soon found out that some of these rocks were evidently much 
newer than the others; these were called “ New Red Sandstone,” 
the rest “Old Red Sandstone.” Subsequent research gradually 
proved that some of the arenaceous rocks included in the latter 
belonged to the Laurentian (?), Cambrian, Cambro-Silurian, 
Silurian, and Carboniferous systems, and by degrees all these 
were taken away from that formation, leaving those still called 
Old Red Sandstone, but which, as I believe, really belong to the 
Silurian and Carboniferous series; these most English geologists 
appear to believe constitute a formation in themselves, though a 
few divide them into Upper or Carboniferous Old Red Sand- 
stone, and Lower or Silurian Old Red Sandstone. 
The “Lower Old Red Sandstone,” and the associated rocks 
that have in them characteristic Silurian fossils, are those which 
I have already called Silurians, and it is to these that I shall now 
refer. 
In Geikie’s paper, already mentioned, it is shown that in Great 
