412 THE CARBONIFEROUS PERIOD 



changes of level, which are reflected in the character of the rocks, 

 and in erosion of the lower strata, forming depressions and basins 

 in which peat bogs gathered. In the first instance, the change 

 consisted in a very extensive but slow elevation of the sea-bottom, 

 raising it in places into land. The coal measures which some- 

 times lie unconformably upon the Lower Carboniferous, were 

 inaugurated by a transgression of the sea, beyond the areas occu- 

 pied by the Lower Carboniferous waters. The thick sheets of 

 conglomerate and coarse sandstones {Millstone Grit) at the base 

 of the coal measures were laid down in the encroaching sea. 

 Contemporaneous erosion (see p. 272) and local unconformities 

 occur within the coal measures. The movement resulted in mak- 

 ing a vast area of low-lying lands, which were raised very little above 

 sea-level and upon which great swamps and marshes were estab- 

 lished, where vegetation flourished in tropical luxuriance. A very 

 slow subsidence, often intermittent, allowed great thicknesses of 

 material to accumulate, but frequently a more rapid sinking 

 brought in the sea, or bodies of fresh water over the bogs, killing 

 the trees which grew there. We cannot yet determine how far 

 the different coal regions represent separate basins, and how far 

 their separation is due to the subsequent removal of connecting 

 strata, but even in connected areas we find great differences in 

 the nature and thickness of the beds. This indicates that oscil- 

 lations of level of different amounts took place in particular parts 

 of the same basin. Thus, in one portion may occur a coal seam 

 of great thickness, divided into two or more layers by exceedingly 

 thin " partings " of shale. As we trace the coal seam in the 

 proper direction, the partings gradually grow thicker, until, 

 perhaps, they become strata, that intervene between very dis- 

 tinct and quite widely separated coal seams, each of which is 

 continuous with the corresponding portion of the thick seam. 

 The meaning of such a structure is, that while one part of the 

 bog subsided very slowly, permitting the almost uninterrupted 

 accumulation of vegetable matter, other portions sank more 

 rapidly and were inundated with water, which deposited mechan- 

 ical sediments on the surface of the submerged bog. 



