408 MK. B. THOMPSON ON LANDSCAPE MARBLE. [Aug. 1 894, 



would afterwards become filled up with other matter in the form of 

 lenticular masses. 1 



After the subsidence of the flood and the resumption of normal 

 conditions, for some time the waters would be more than usually 

 free from organic matter, owing to the washing that the land had 

 previously received, and the sediment would consequently make 

 purer limestone (8). 2 



Let us now consider what is happening in the dark layer whilst 

 this purer limestone is being deposited. The thicker dark layer 

 produced by the supposed flood would consist of vegetable matter 

 less decomposed than that normally brought down, and so under the 

 warm shallow water of the estuary it would continue to decompose, 

 and, like the black ooze from a dirty pond, give off bubbles of gas. 



At first the bubbles passed off freely ; consequently they were not 

 very large, and having little or no sediment to lift could come off 

 anywhere, and so a great many spurts of dark matter were formed (8). 

 As the calcareous sediment increased in thickness over the organic 

 layer the bubbles of gas escaped less frequently, from a smaller 

 number of points, and were larger ; for, before they could free 

 themselves from the sediment, they must have attained sufficient 

 buoyancy to displace the material above them. 



The bubbles in rising gave rise to three of the phenomena observed 

 in Landscape Marble : — (a) they lifted somewhat the calcareous 

 matter contiguous to their path (8) ; (b) they relieved the pressure 

 behind, so that the dark matter which gave rise to them followed 

 more or less readily according to its plasticity ; and (c) the white 

 calcareous matter furnished the necessary pressure to cause the dark 

 matter to rise, and as a consequence filled the place from which the 

 latter was driven, thus becoming depressed between the arborescent 

 markings. 



As sediment increased and tubes of organic matter were formed 

 in it, these latter themselves became centres for the evolution of gas, 

 and, having less material to lift, would discharge it more often, in 

 deviating paths — mostly, perhaps, in the first instance, through 

 relief of pressure at the side just when a large bubble escaped from 

 a neighbouring vent. 



After a considerable time the decomposition of the organic matter 

 would be approaching completion, and three results would follow : — 

 (a) the organic matter left would consist chiefly of finely divided 

 carbon ; (6) the bubbles of gas would be few, large, and far between ; 

 and (c) when a bubble did escape it would produce vortex rings in 

 the water above, sufficiently powerful to disturb and re-arrange the 

 last layers of calcareous sediment, and well mix it with the black 

 carbon now filling and escaping from the tubes (9). (Layer F, 

 figs. 1 & 2.) 



Ultimately the escape of gas ceased, and layers similar to those 



1 The specimen shown in fig. 2 (p. 397) appears to have been so situated that 

 it was peculiarly subject to disturbance ; both the lower and upper layers appear 

 to have undergone re-arrangement of material. 



2 The matrix in which the arborescent markings occur is much lighter than 

 •any other part of the stone. 



