46 
HUGHES : INGLEBOROUGH. 
on to a mud flat. So in the rocks Trinucleus concentricus is 
found in shale, but Trinucleus seticornis more commonly in 
sandy beds ; yet they are contemporary and are often found 
together. When the sand was carried over the mudflats where 
certain GraptoUtes thrived they had to go or die, but when 
mud settled there again they or their representatives returned. 
Some of the common Silurian fossils, such as the smooth 
Pentameri, have a very short range in time, others, as Cardiola 
interrupia, run right through the greater part of the Silurian. 
Therefore when we are applying the palseontological vernier 
to our scale of rocks we have various problems to solve before 
we can trust the first results at which we appear to have arrived, 
especially when we are making use of negative evidence. When 
we fail to find the characteristic fossils of an otherwise well- 
marked horizon, we must inquire whether the deposits in which 
those fossils were found have under locally varying con- 
ditions of sedimentation gradually changed from coarse to fine 
or fine to coarse, so that synchronous deposits carrying different 
forms of life occur in adjacent areas. Sometimes we can explain 
the absence of the fossils we expected to find by supposing 
that, without any discordance, sedimentation ceased over the 
area under examination, so that the deposit in which those 
fossils do occur has no equivalent in time in the area under 
examination, but has thinned out to nothing before it reached 
it. This of course implies that beds representing it, or part 
of it, may be expected in the intermediate area. 
We should, for the practical working out of the geology of 
a district, classify by the more obvious petrographical characters, 
and then identify by palaeontology, but petrology and palae- 
ontology should each be kept subordinate, as the handmaid, 
not the mistress, of stratigraphy. 
In the preceding part of this paper I discussed the question 
of what we should take as the base of the Silurian, and gave 
a general sketch of the whole series, "with a view to determining 
what part is represented under Ingleborough. I now proceed 
to examine these in greater detail. 
Above the irregular and ill- defined deposit which we have 
spoken of as the basement bed, we found thin variable bands, 
