MARIN-E FOSSILS IN THE COAL-MEASFEES OF FIFE. 753 



Mollusca, Cypridce, and plants of the latter indicate freshwater 

 conditions, it is evident that such conditions were occasionally 

 overborne by inroads of the sea, bringing back species of shells and 

 crinoids that had existed in the Carboniferous-Limestone ocean of 

 an earlier period. This appears to have taken place in the areas of 

 most coal-fields, and repeatedly in some. It is thus reasonable to 

 assume that the open sea was not far off when the British Coal- 

 measures were being formed, and that a slight increase in the rate of 

 depression of the area sufficed to bring back the sea and marine life. 



There is undoubtedly something peculiar about the ordinary 

 fauna of the Coal-measures, though the peculiarity is, perhaps, just 

 as great, whether it is viewed as of freshwater or of marine origin. 

 And though it cannot have been marine in the same sense as the 

 fauna of the Carboniferous Limestone or any open-sea deposit, it can 

 scarcely be understood on the view of its being of lacustrine origin, as 

 some geologists still hold. Certainly these intercalated marine beds 

 seem easier of explanation when the formation is looked upon as the 

 deltaic or, in some way, marginal accumulations of a large land-area. 

 Under such conditions everything observed in the palaeontology of 

 the strata can be accounted for, whether the indications be of 

 dense vegetable growth, vegetable drift, or of freshwater, brackish- 

 water, or open-sea animal life. 



Anyone who has studied the Carboniferous series of Eife strati- 

 graphically, from the base of the Calciferous Sandstone* upward, 

 will only see in these marine beds the last and final instances of 

 what has come under his notice times out of number before, the 

 coming in of marine deposits in succession to shales, sandstones, 

 fire-clays, and coals containing plant remains or estuarine fossils. 

 The whole formation indicates a long series of depressions with 

 intervening siltings up during periods of rest, the former often 

 bringing in marine conditions, the latter as often resulting in an 

 approach to land-surfaces and subterrestrial conditions. This is 

 true of the Calciferous Sandstones, where there are more coals (only 

 poor and thin) than in the Coal-measures proper, but where thin 

 limestones and other marine strata are comparatively common. In 

 the lower portion of the Carboniferous-Limestone series marine beds 

 are thicker and the remains of marine life more abundant, though 

 coals and plant-bearing beds come in among them. The same is the 

 case, though less pronounced, in the upper portion of the Carboni- 

 ferous Limestone; while between the upper and lower portions 

 there is a thick group of carbonaceous strata containing as good 

 "workable coals, and as many of them, as exist in the true Coal- 

 measures one thousand feet or so higher up. And so in the group 

 of strata classed as Millstone Grit, marine beds alternate with others 

 containing vegetable remains and poor coals. Then follow the 

 Coal-measures with the second great series of thick coals, with here 

 and there marine beds, without the least indications of unconformity 

 or physical break. In fact there is no such break anywhere in the 

 Carboniferous series of Fife. The whole succession is one of regular 

 * Quart, Journ. Geol. See. vol. xxxvi. p. 559 &c. 



