DAVIS: FISH-REMAIN S IN THE COAL-MEASURES. 45 
to the bottom and be as evenly and perfectly fossilized as 
though they had been carefully pressed for an herbarium. 
The most delicate specimens are always found in the shales 
composed of mud of the finest kind, and would consequently 
have the greatest distance to travel before reaching their 
destination, and pass through proportionate liability to 
destruction. In the coarser kinds of shale, well-preserved 
fronds are rare, and in the sandstones nearly or quite absent. 
The evidence afforded by the mollusca confirms the Fresh- water 
theory of deposition. In the case more especially under 
consideration of the Adwalton Stone Coal, there is, imme- 
diately above the cannel coal and " hubb," a bed of ironstone, 
four inches thick, containing Unios or Anthracosia ; and then 
a bed, several inches thick, composed entirely of the remains 
of Anthracosia — higher up they become mixed with a black 
shale, two or three feet in thickness, and are fewer in number 
in the upper part. Above the black shale there occurs some 
twentj' feet of whitish and blue shales, containing numerous 
bands of ironstone nodules. Shells of Anthracosia are 
common in the ironstone, but do not occur in the shale. 
This is exactly the state of things that might be expected 
to have taken place. After the accumulation of decaying 
vegetable matter, which formed the cannel or stone coal, 
there was a period during which a large percentage of mud 
was brought with the vegetable refuse, and being deposited 
together, the impure cannel coal, locally termed " hubb," was 
formed. Afterwards, from some cause, the amount of 
vegetable matter became less, and the muddy bottoms of the 
pools were thronged by immense numbers of molluscs, with 
thin bivalve shells, the fossil remains of which are always 
found more or less crushed. They are such shells as would 
be found at the present time inhabiting and luxuriating in 
semi-stagnant pools, weak and thin; sufficient to serve for the 
protection of the mollusc in a mass of soft mud, but totally 
