Vol. 50.] ME. B. THOMPSON ON LANDSCAPE MARBLE. 405 



The alteration of colour in the outer layers of some specimens 

 ■appears to be a fairly common characteristic (7, 9, 12), for Edward 

 Owen several times refers to it {op. cit. pp. 169, 175) ; though 

 whether they are like this when first quarried or only become so 

 on exposure I cannot say : one would think the latter is the case. 

 The only interest attaching to this very common feature is that, 

 under the microscope, the change of colour appears to be confined 

 to the aragonite ; at least, it is so in my specimen (fig. 1, p. 395). 

 Prom this it would appear that any carbon present was slowly 

 oxidized, and iron and manganese only left as colouring matter, a 

 sufficient amount of these being present to colour the nearly trans- 

 parent calcium carbonate a light brown, although not enough to 

 colour the thin, opaque, white layer produced on ignition. There 

 is no difficulty in supposing these changes to have occurred, for the 

 discolouration some distance into the stone (7, 9, 12) shows the 

 possibility of water and dissolved gases penetrating thus far. 



The thin veins of calc-spar (13) may have been produced at any 

 time posterior to the formation of the bed by ordinary desiccation 

 and subsequent infiltration. 



The chemical composition does not suggest anything of importance 

 besides what I have already dealt with ; the phosphoric acid found 

 in the stone may, or may not, all have been introduced by the plants. 

 Some of it almost certainly was. 



VIII. Experiments made to reproduce the Characteristics oe 

 the Landscape Marble. 



Supposing the explanations just offered of the origin of the 

 peculiar features of Landscape Marble to be true, it seemed probable 

 that they might be reproduced artificially. I therefore made the 

 following experiments. 



(1) A rather wide glass tube was plugged at one end with a 

 mixture of fine sand and plaster of Paris ; over this was spread a 

 layer of ground chalk rendered dark by admixture with oxide of man- 

 ganese, and on the top of all some very fine sand. The whole was 

 sufficiently porous to let water through gradually. On pouring dilute 

 hydrochloric acid into the tube effervescence occurred in the middle 

 dark layer, and bubbles escaped from the top, but there was no 

 noticeable rising of the dark layer to follow the bubbles of carbonic- 

 acid gas. It was not possible to get out the triple plug without 

 breaking it up, and another plan was adopted. 



(2) The same materials were used as in the previous experiment, 

 but they were placed in a small flower-pot. The whole experiment 

 was conducted more slowly, and after its probable completion a 

 solution of carbonate of soda was run through, with the idea that 

 some calcium carbonate would be re-formed, which would help to fix 

 any characters that had been developed. On taking the material 

 out of the flower-pot, I noticed (a) that the layers separated the 

 one from the other ; (b) that the dark matter had not risen into the 



