406 MR. B. THOMPSON ON LANDSCAPE MARBLE. [Aug. 1 894, 



upper layer, although (c) the latter was full of tubes running in 

 various directions (but seldom straight up) through which the car- 

 bonic acid had escaped ; (d) that the tubes were comparable in size 

 with the dark arborescent markings of Landscape Marble, though on 

 the whole, perhaps, a little larger ; and (e) that the upper surface of 

 the upper layer was thrown into hills and hollows, with a small hole 

 at the centre of each depression, through which no doubt the last 

 bubbles of carbonic-acid gas had escaped as the material was setting 

 or drying. 



(3) A glass tank was made, such as is used for showing chemical 

 reactions on a screen by means of a magic lantern, but made to leak 

 just a little at two opposite corners at the bottom. The lower part 

 was filled with very fine siliceous matter deposited from water, the 

 middle with precipitated calcium carbonate and carbon in the form of 

 lamp-black (both mixed well together before being poured in), and 

 over these some more fine siliceous matter. All being ready, dilute 

 hydrochloric acid was poured into the space above the various 

 sediments, but they now seemed to be impervious, and nothing 

 happened. When I made a connexion between the hydrochloric 

 acid and the dark layer by means of a long needle, the evolution of 

 gas was violent, and there was something like a miniature volcanic 

 eruption. Although not all that I hoped, two additional points of 

 evidence were gained : (a) there could be no mistake about the 

 carbon rising with the gas now — in fact, it was carried up so much, 

 and mixed with siliceous material from the sides of the vent, as to 

 form a half-cone, as it were, on each side of the crater ; and (b) a 

 mixed layer was produced above the siliceous sediment, in which the 

 dark matter (carbon) largely predominated. 



I may mention that experiments in test-tubes generally failed, 

 because any slowly generated carbonic acid would lift a plug of even 

 rather coarse wet sand right out of the tube rather than bubble 

 through it. The production of a dome seemed a necessary pre- 

 liminary to breaking through, and in a test-tube the small superficial 

 area in proportion to thickness of sediment did not admit of dome- 

 formation. 



Thus all the main features of the Cotham Stone, with the ex- 

 ception of the differences of mineral character and colour, have been 

 reproduced artificially. 



IX. Conditions under which the Landscape Marble was formed. 



To sketch imaginative pictures is perhaps more interesting than 

 the investigation of dry facts, but sometimes less profitable ; and 

 therefore, although I am about to draw a picture of the conditions 

 under "which the Cotham Stone was deposited — as I seem to see it — 

 I do not attach as much importance to this part of the paper as to 

 the preceding pages. 



Conceive a broad estuary receiving the drainage and fine sedi- 

 ment of a considerable area of flat fenland, by means of sluggish 

 rivers rising in somewhat higher calcareous lands also covered with 



