INTRODUCTION. 5 



Various attempts at classification have been made in tbe case of Carboni- 

 ferous strata, but with no very satisfactory result. For while it is easy to 

 classify the rocks which represent, in any area, the Carboniferous beds, it 

 is with great difficulty that any decision can be arrived at as to the exact 

 equivalents of the beds in two series of rocks of different areas. It appears 

 to ilie, from lithological and palseontological points of view, that the rocks which 

 represent the Carboniferous period in Northumberland and Scotland were laid 

 down on a slowly sinking floor in very shallow water, at times probably fresh or 

 very slightly brackish, while occasionally depression went on more rapidly and 

 deeper marine conditions obtained. In the Derbyshire area, at first, a steady 

 marine deposition was going on at some considerable depth ; but as time went 

 on the land sank less rapidly, the sea basin became almost filled, the shallow 

 water or shore beds extended further south, and the continuous formation of 

 calcareous ooze was replaced by the deposition of the muds and sands brought 

 down by river and other agents of surface denudation, not permanently at first, 

 owing to oscillations in the rate of the lowering of the land. Little can be known 

 of the comparative rates of the deposit of the alternating beds of the Calciferous 

 Sandstone group and the Mountain Limestone, because, on the one hand, the 

 succession and repetition of strata point to varied rates of deposit, and the 

 rich fossil flora and seams of coal entombed in the rocks indicate intervals in 

 the deposit, and it is impossible to gauge the length of time during which the 

 terrestrial conditions obtained. From a palfeontological point- of view, deposits of 

 varied depth would naturally contain faunas of different aspect, and therefore 

 fossils can hardly be relied upon to indicate stratigraphical Equivalents, though they 

 may, in any area, mark out zones or horizons ; for as the depth or clearness of the 

 water and condition of the bottom altered, new forms would migrate; and thus in one 

 locality certain forms would be found in horizontal sequence to others (and conse- 

 quently, from the point of view of time not contemporaneous), which had been living 

 contemporaneously in diflbrent areas : and thus in the Anglo-Scottish Carboniferous 

 area, as the sea deepened, southern forms would extend northward, and be 

 intercalated between beds containing shallower fauna ; while as the basin filled up 

 the shallow water fauna would migrate south, and overlie that which was 

 characteristic of deeper water. In the Calciferous Sandstone series of the 

 Fifeshire coast it would appear that alternating conditions were repeated ; such 

 lithological repetitions being accompanied by the fresh invasion of a fauna which 

 had been driven away from the area by an unsuitable environment, but which had 

 evidently survived in some other area, and was able, when suitable conditions 

 reappeared, to migrate back again, but at a higher level, to its old haunts. 

 Certain species, it is true, appear to have been able to survive under very varied 

 conditions; but a close examination will show a great variation in size — for 



