410 MR. B. THOMPSON ON LANDSCAPE MARBLE. [Aug. 1894, 



of organic and inorganic matter might result in as great an evolution 

 of gas and uplifting of the layers above, although without being able 

 to produce arborescent markings. 



Between the top layer of the White Lias — the Sun-bed — and 

 Gotham Stone there are otber beds presenting corrugated surfaces in 

 homogeneous limestone. These, I should imagine, contained organic 

 matter, although, on account of its being evenly disseminated in the 

 rock, its previous presence is not noticeable. 



Discussion. 



Mr. H. B. Woodward remarked that the Landscape Marble 

 occurred at the junction of the black Avicula contorta-shales and 

 the White Lias, and exhibited a commingling of dark argillaceous 

 with calcareous sediment. Where the bed was persistent, it was 

 simply banded limestone ; where the arborescent markings were 

 present, the bed occurred in nodular and isolated masses, sometimes 

 4 feet or more across, and with the characteristic crinkly surface. 

 Thus he had concluded that the arborescent markings were produced 

 by irregular changes in the dark and pale muds during the solidifi- 

 cation of the stone. In illustration of the connexion between 

 arborescent markings and nodular and crinkly stone, he exhibited 

 specimens from the Purbeck Beds, and also from the Estheria-hed 

 (Rhaetic) of Westbury-on-Severn. The chemical and physical 

 aspects of the subject he could not discuss, and that had now been 

 ably dealt with by Mr. Thompson. 



Prof. T. Rupert Jones drew attention to what he thought was 

 the Rev. Osmond Fisher's suggestion of the cause of the arborescent 

 markings in the Landscape Marble. Among successive layers of mud 

 and tufa, in a marshy district, some of the thicker layers of rotten 

 plants gave way under the superincumbent tufa, which broke down, 

 and the carbonaceous mud was forced up along the lines of breakage. 

 The speaker thought that, by combining this with other hypotheses 

 already alluded to, we might find a true cause of the peculiar 

 markings in the stone. 



Mr. F. A. Bather asked whether the drawing represented the 

 specimens the same way up as in situ. 



Mr. Monckton said there could be no doubt that the mammillated 

 surface was at the top. 



The Author said that he first of all desired to thank Mr. H. B. 

 Woodward for his forethought and kindness in exhibiting, by per- 

 mission of the Director-General of the Geological Survey, various 

 specimens of Landscape Marble and Purbeck Stone showing arbor- 

 escent and allied markings, in illustration of the paper. In reply 

 to Mr. Woodward's remarks he said that the chief reason for not 

 accepting his explanation of the arborescent markings was that the 

 shrinkage of the stone had not affected the contiguous thinner dark 

 bands ; although shrinkage of the upper layers had aided in the 

 formation of the corrugated upper surface. He also replied to 

 several questions, and more fully explained the results obtained in 

 experiments made for the artificial production of the essential and 

 characteristic features of the stone. 



