
334 STRUCTURAL AND FIELD GEOLOGY 
The occurrence of numerous partings of shale in certain coal-seams, 
and the fact that such partings thicken-out in many cases so as eventu- 
ally to separate one coal-seam into two or more seams, divided the one 
from the other by many feet or yards of sedimentary strata—are hardly 
consistent with the theory of growth zz sztu. Nor does this theory 
account satisfactorily for the well-known fact that individual coal-seams 
often consist of different kinds of coal, occurring one above the other in 
irregular lenticular layers, or in more or less persistent seams. For 
example, many common coals contain interlaminations of cannel coal, 
which may be an inch or two in thickness, while some layers of certain 
common coals are more bituminous than others. Further, it may be 
noted that fish-teeth, and shells of what were either fresh-water or 
brackish-water molluscs, now and again occur in coals or in the shales 
immediately associated with them ; even marine shells, as we have seen, 
occasionally appear in connection with coals. In the Scottish coal-fields, 
indeed, thin coals not infrequently are underlaid or overlaid directly with 
calcareous shales and thin limestones, throughout which marine organic 
remains abound. These phenomena are very hard to explain in any 
other way than by supposing that such coals were accumulated in water 
—that, in short, they are sedimentary formations. It has been objected 
that the comparative purity of coal—the relative absence of sand and 
mud—is against the supposition that coal could have been accumulated 
by drift. It is obvious, however, that vegetable matter carried down by 
streams and rivers into lakes and estuaries does not necessarily become 
impregnated or mixed up with ordinary sediment. When a river enters 
an estuary, its sediments become sifted out—the finer ingredients being 
spread over the widest area. Should the river carry rafts and sheets of 
vegetable débris, these may well be transported far beyond the reach of 
ordinary sediment. Becoming waterlogged, this vegetable débris would 
eventually sink, and might therefore come to rest in regions rarely or 
never reached by ordinary sedimentary matter. It is quite conceivable, 
therefore, that over the floors of estuaries and broad bays, at some 
distance from the land, vast accumulations of vegetable detritus might 
take place. The interosculation of such sheets of vegetable matter with 
sand and clay is just what one would expect to occur, while the occasional 
appearance of fresh water, estuarine, or marine organisms need not 
surprise us, for the accumulation of vegetable débris might take place 
either in large rivers, in lakes, in estuaries, or in more open bays of the 
sea. It is obvious, moreover, that the drift hypothesis offers a feasible 
explanation of the variable character of many coals, and accounts well 
enough for the frequent intercalation in the sandstones of thin lenticular 
layers and beds of coal and coaly matter. 
Although the observer may have assured himself that 
only thin coals crop out at the surface, he need not, on that 
account, conclude that further research is useless. It is quite 
possible that the seams seen in actual section may thicken- 

