Vol. 50.] ME. B. THOMPSON ON LANDSCAPE MARBLE. 399 



few seconds by directing a jet of flame on to the dark part by means 

 of a blowpipe. 1 



Y. Its Mode oe Occurrence. 



16. The stone occurs in isolated and lenticular masses which are 

 sometimes less than 1 foot across, sometimes 3 or 4 feet. A stone 

 some 2 feet in diameter or length is usually 8 or 9 inches thick 

 (Edward Owen, p. 172). 



The stratigraphical equivalent of the Cotham Stone occurs in 

 other places as a fairly persistent layer ; where this is so it is 

 oanded and evenly bedded, but the arborescent markings and crinkly 

 upper surface are wanting. Between the ordinary banded limestone 

 and the distinctly arborescent types many intermediate varieties 

 may be found. 



17. The stone is not very continuous, even where it takes the bed 

 form, although met with more or less over a large area. It may be 

 that at some places the bed is present, but not identified, because of 

 the absence of the arborescent markings. 



18. The position of the Cotham Stone is in the Rhoitic Beds, at or 

 near the junction of the black Avicula contorta-ah&les with tho 

 overlying beds of "White Lias, that is, between dark argillaceous sedi- 

 ment and almost pure calcareous mud, though in some localities a 

 few inches of dark clay may be found above the stone. 



VI. Theories as to the Origin op the Landscape Marble. 



Edward Owen, who first described the Landscape Marble, thought 

 that the arborescent markings were produced by the escape of im- 

 prisoned air. Some have thought that gaseous emanations from the 

 black mud of the Avicida contorta-shales had something to do with 

 the peculiar structure of the marble. Others again have considered 

 that infiltration of dark mineral matter would account for it. 



Mr. H. B. Woodward's recent explanation of the origin of the 

 Landscape Marble 2 is as follows : — " It appears to me that the 

 arborescent markings were produced during the consolidation of 

 the stone, and more particularly by the shrinking of its upper 

 portions. In this way, and while the mud was still in a more or 

 less pasty condition, one or more of the dark films in the banded mass 

 were disarranged and dispersed in arborescent form in the slowly 

 setting rock .... It may be that the production of the isolated 

 masses of rock, with their irregular upper surfaces, was attended by 

 some pause in the deposition of sediment, and by exposure of the 

 layers to the sun's rays . . . The process of formation of the Land- 

 scape Marble seems to me to have been mainly mechanical, although, 



1 Since writing the above I have read the following passage : — ' Mr. Allan 

 Dick, who has kindly examined a specimen of the Ootham Marble, tells me 

 that dark portions of the stone are not due to the presence of manganese- or 

 iron-ores, but are probably clue to carbonaceous matter.' — Mem. Geol. Surv. 

 1893, ' The Jurassic Rocks of Britain,' by H. B. Woodward, vol. iii. p. 31. 



2 Geol. Mag. 1892, pp. 112, 114. 



2e2 



