184 THE CARBONIFEROUS SYSTEM. 



the contrary, except in the occurrence of one underclay overlaid by 

 coaly shale, these thirty-five feet of sand and mud may have been 

 rapidly deposited. It is quite possible that the formation of the one- 

 inch of coal in the bottom of the group may have occupied a longer 

 time than the deposition of the whole of the other beds. The reader 

 will note here that the absolute thickness of any bed or mass of beds 

 is no measure of the time occupied in their formation. A layer of 

 sand may be spread over a wide surface by a single storm or inundation, 

 but it requires time to accumulate even a very inconsiderable amount 

 of organic matter by the slow growth of animals or plants. 



The next Subdivision, No. III., including Coal-groups 40, 41, and 

 42, is very similar to No. I. It shows that the locality was again for 

 a very long period alternately a swamp and a lagoon. I say for a 

 very long time, for much of this group consists of bituminous lime- 

 stone, Naiadites shales or "mussel beds," and coal, all beds requiring 

 a veiy long lapse of time for their growth. There is every reason, 

 for example, to believe that the three-feet bed of bituminous lime- 

 stone, nearly in the middle of the group, consists mainly of the 

 remains of Cythere, fish, and other aquatic creatures, accumulated 

 by the slow growth of successive generations ; and if we have any 

 idea of the growth of modern beds of this kind, we will be disposed 

 to measure the growth of this limestone by centuries. 



Subdivision IV. is comparatively barren of organic remains, and 

 consists of coarse mechanical detritus ; and it will be observed that 

 throughout the section groups of beds of this kind alternate with 

 others composed of fine silt and organic matter. There were, in other 

 words, long-continued swamp and lagoon periods, alternating with 

 periods in which the waters of the sea or turbid streams were bearing 

 in sand and mud. We have here, however, two surfaces which had 

 sufficient permanence, as land or swamp soils, to support trees and 

 Calamites. 



In Subdivision V. we have a recurrence, on a small scale, of the 

 conditions of Subdivision III. 



Subdivision VI. is another great series of sandstones and chocolate- 

 coloured shales. It has, however, one erect tree, probably a Sig Maria, 

 rooted in an underclay with Stigmaria rootlets ; and in the lowest 

 sandstone there is a great mass of prostrate trunks of trees, imperfectly 

 preserved, probably the wreck of some land-flood, or the drift-wood 

 from a forest-clad coast. 



In Subdivision VII. the beds present an order similar to that in 

 the first group, and which we shall find frequently repeated in the 

 section. First, we have an underclay, a soil on which grew a small 



