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" coal formation, and the final cause of their predominance 
" in that period, would seem from numerous facts to be, that 
" by their assimilation of the carbon, and liberation of the 
" oxygen with which it was combined, they might purify the 
" atmosphere, and bring it into a condition in which it would 
" become respirable by reptiles, beasts, and man. 
" That such was the primitive condition of the atmosphere, 
" and that it was thus gradually purified by the growth of 
" plants, seems to be not improbable, from the circumstance 
" that reptiles and other cold-blooded animals which can 
" endure and enjoy an atmosphere that would be fatal to 
" warm-blooded animals and man, are the earliest of which 
any fossil remains are found. 
" That the atmosphere at first was very greatly loaded 
" with carbonic acid, is probable from reptiles not appearing 
" until after the coal formation^ 
I do not wholly protest against the hypothesis that carbonic 
acid gas was much more prevalent at that period than now ; 
but I submit that these foot-prints tend materially to qualify 
the ingenious supposition of men very eminent for their 
attainments in natural science ; and, I think, it will be con- 
ceded that they have an important bearing upon the condi- 
tion of the atmosphere during the deposition of the coal 
strata. 
Being at Brighouse in the year 1836, I visited the 
celebrated flag quarries of that vicinity, and in one quarry, 
between Brighouse and Rastrick, had the unexpected pleasure 
of finding some large slabs of stone covered with foot-prints 
of a small animal. Many of the tracks formed right lines 
up and down the surface, while some of them diverged across 
in an oblique direction to the right and left, the extreme 
angles formed not being greater than about 60°; proving 
quite satisfactorily that the animals had walked up and 
down a sloping beach of fine sand, to and from the water. 
