DAKYNS : GLACIAL BEDS AT BRIDLINGTON. 
127 
polished stones, has been set down as no true boulder clay, 
but a mere mass of stuff washed out of the boulder clay. 
And as part of the story, a portentous number of black 
arrows, pointing now up, now down, are introduced to 
indicate so many upheavals and depressions of the land. 
One might fancy we had to do with cases like that lately 
figured by the Society in their photograph of Ribblesdale, 
where massive beds of mountain limestone are seen lying 
nearly flat on the upturned and denuded edges of Old 
Silurian Slates. Whereas the truth is that in the glacial 
beds, unconformities (so called) and eroded surfaces, instead 
of indicating great breaks in the series, are, in nine cases out 
of ten, merely local and contemporaneous. Erosion was the 
order of the day. Each new extrusion of the ice-sheet 
ploughed out a portion of the previously deposited beds, and 
the hollow was anon filled in with a fresh deposit, whether 
of current-borne sands and clays or of ice-borne till. But it is 
all one great boulder clay formation. Of course there are 
wide-spread divisions, implying great lapse of time, and 
great changes of condition ; such as the two boulder clays 
(very distinct in character) of Lancashire, with the far- 
reaching " middle sands" between them. But it is not every 
wretched patch of gravel, or every eroded surface of a till 
that has this meaning. 
Again, too much faith is apt to be placed in litho- 
logical differences as indicating differences of ao-e, as if 
one and the same bed of till would not necessarily have 
different sets of stones embedded in it in different parts 
of its extent. Lastly, the same may be said about colour 
as a test of age. There are cases where colour is an in- 
valuable test, as in some of the Thanet beds, and in the tea- 
green shale of the Rhcetics ; but before colour can be safely 
so used (save quite locally), it must be 'proved, not assumed, 
to be a characteristic mark ; and if all colours do take among 
