18 Proceedings of the Royal Physical Society. 
Riist, to whom I had sent specimens, wrote to me in reply 
saying that he saw no reason to doubt that the rock was a 
genuine Radiolarian ooze. 
It may be objected here that the association of these 
Nadiolarian cherts with beds containing Graptolites would 
go to show that the former cannot be of deep-water origin. 
Apart, however, from the fact that we know of no case in 
which a “pure” Radiolarian deposit has been formed in 
shallow water, it may well be contended that the association 
with Graptolitic beds is really an additional corroboration 
of the view that these Ordovician cherts are deep-water 
accumulations, All the facts which we know about 
Graptolites would lead us to believe that the normal forms 
of the group, at any rate, were essentially pelagic organisms. 
It is only upon this supposition that we can rationally ex- 
plain the well-known facts as to the restriction of particular 
species of Graptolites to special, and often exceedingly thin 
zones of rock, and the extraordinarily wide diffusion of 
particular types of the group in beds (often of very limited 
thickness) at identical horizons over areas often geographic- 
ally very remote. This argument is, of course, quite 
unaffected by the occasional occurrence of Graptolites in 
rocks of undoubted shallow-water origin, since pelagic 
organisms may be, and often are, driven into the vicinity of 
land by currents or winds. Apart from Graptolites, the only 
organisms found in the beds directly associated with the 
Ordovician cherts under consideration are a few minute 
Brachiopods and some small Crustaceans belonging to the 
order of the Phyllocarida; and both of these may well have 
inhabited deep water. 
It might, in fact, be contended with considerable prob- 
ability that a distinctively Graptolitiferous series, such 
as the “Moffat series” of the south of Scotland, or the 
“Stockdale series” of the north of England, is essentially 
a deep-water deposit—even though it were not associated 
with Radiolarian cherts. It is all but certain, as above said, 
that the majority of the Graptolites were pelagic organisms, 
and that their remains are therefore most likely to be 
found in deep-water deposits; though, under suitable 
