Yol. 50.] MR. B. THOMPSON ON LANDSCAPE MAKBLE. 409 



found at the base of the stone were formed, though even then the 

 production of gas had not entirely ceased, and so the last layers of 

 plastic matter are uplifted in places, thus producing an uneven or 

 mammillated surface. To this result no doubt contraction of the 

 stone on setting contributed, particularly as it had a partly organic 

 and partly gaseous nucleus. 



The subsequent changes, which resulted in the formation of ara- 

 gonite in some cases, and finer crystals of calcite only in others, must 

 be left unexplained for the present. We know that there would be 

 an aqueous solution of carbonic-acid gas under gradually increasing 

 pressure, and in contact with calcium carbonate which it is capable of 

 dissolving. 



If the explanation which I have just given of the origin of 

 Landscape Marble be correct, we ought sometimes to find it in other 

 rocks and at other places where like conditions prevailed ; and there 

 ought to be grades of development. This really is the case (16, 17). 

 The Esther ia-bed in the upper part of the Rhaetic Beds at Garden 

 Cliff and Westbury-on-Severn, and several others, are mentioned by 

 Mr. H. B. Woodward as instances. 1 



One might be inclined to ask why we do not find these markings in 

 the sands and shales above coal-seams. Well, several circumstances 

 might prevent that. The sedimentation might have been too rapid, so 

 that the bubbles of gas were mostly imprisoned ; or the material might 

 have been coarse enough to let the gases pass between the particles 

 without disturbance of the latter. But the chief reason, I suspect, 

 was that the deposition of the shales or sands was not tranquil 

 enough ; a very slight movement of the water in contact with sedi- 

 ment would facilitate the escape of gases and obliterate all trace of 

 arborescent markings. It may be, however, that these markings 

 might be found above some coal-seams — if specially looked for. 



To the questions — (a) Can the arborescent markings occur with- 

 out the corrugated upper surface ? and (6) Can the corrugated surface 

 occur without the markings ? — I should return in the case of the 

 former a negative, and in the case of the latter an affirmative 

 answer, for although a decomposition of organic matter and evolution 

 of gas sufficient to produce the arborescent markings could scarcely 

 fail to affect the sediment deposited just above, an intimate mixture 



1 When this paper was nearly completed, through the kindness of Mr. H. 

 B. Woodward, I was permitted to see a specimen of limestone from the Purbeck 

 strata of Swanage (Durleston Bay), which in several respects resembles the 

 Cotham Landscape Marble. It is very similar in colour to one of my specimens. 

 It contains peculiar dome-shaped markings, starting from and only above a 

 distinct, sharply-defined, dark layer of the same colour ; the upper surface is 

 extremely wrinkled, similar to Landscape Marble, only finer than the specimens 

 of the latter that I have seen, and the bumps seem to correspond with the upper- 

 most domes of darker matter. It differs from Gotham Stone in being of coarser 

 granular structure, of different fracture (not conchoiclal) ; the markings are of a 

 brown colour instead of being almost black, and they reach the surface every- 

 where, the thickness of the part of the stone in which tbey occur being less than 

 ^ inch. I certainly think that this Purbeck stone had an origin similar to 

 that of the Cotham Stone, but, as the gases never had any difficulty in escaping, 

 decomposition was more complete — hence greater absence of carbon and more 

 general diffusion of the markings. 



