394 THE CARBONIFEROUS SYSTEM. 



marl underlying the gypsum. In some parts it is greenish and homo- 

 geneous in texture ; in other parts it is brecciated, and some layers 

 have a brownish colour and shaly texture. In some parts it is highly 

 gypseous and contains layers of granular gypsum, one of which is 

 black, its colour being due to a small proportion of coaly or bituminous 

 matter. 



" (8.) Beyond the marl the shore is occupied for a short space by 

 boulder clay. Beyond this it shows a great thickness of dark shales 

 with calcareous bands, containing a few small shells belonging to the 

 curious little crustacean, Leaia Leydii., represented in Fig. 78 e above. 

 They dip to the E.S.E. at a high angle, and overlie the gypsum. 

 They are succeeded by a thick band of very hard gray and brownish 

 sandstones and shales, containing a few fragments of plants stained 

 with carbonate of copper. These are again overlaid by dark shales, 

 and these by an enormous thickness of gray and brown sandstone 

 and shale. Some of the shales in this part of the section have assumed 

 a kind of slaty or rather prismatic structure." 



I beg the reader to observe, in the above section, the contrast 

 between the hardened sandstones and shales and the soft marls and 

 gypsum, a contrast equally marked in other parts of the Carboniferous 

 districts, and often producing, by the removal of the softer beds, that 

 isolated position of the gypsum masses which is frequently so per- 

 plexing. It is also important to observe, that this great mass of 

 gypsum is a regular bed, interstratified with the others, and belonging 

 to the series of processes by which the whole were formed. I have 

 already, in noticing the gypsum of Windsor, referred to its probable 

 origin, and may now apply the same method of explanation to that 

 of Plaister Cove. On this view, then, the history of this deposit will 

 be as follows : — 



First, The accumulation of a vast number of very thin layers of 

 limestone, either so rapidly or at so great a depth that organic remains 

 were not included in any except the latest layers. Secondly, The 

 introduction of sulphuric acid, either in aqueous solution or in the 

 form of vapour ; the acid being a product of the volcanic action whose 

 evidences remain in the neighbouring hills. At first the quantity of 

 acid was too small, or the breadth of sea through which it was diflfused 

 too great, to prevent the deposition of much carbonate of lime along 

 with the gypsum produced ; and its introduction was accompanied 

 by the accumulation on the sea-bottom of a greater quantity of me- 

 chanical detritus than formerly : hence the first consequence of the 

 change was the deposition of gypseous marl. At this stage organic 

 matter was present, either in the sea or the detritus deposited, in 



