Vol. 50.] MB. B. THOMPSON ON LANDSCAPE MARBLE. 401 



coloured matter. 1 The darker matter must necessarily have been 

 more fluid than, and quite as finely constituted as the matrix to 

 have been displaced by and dispersed in the latter, and then a kind 

 of marking to which the term ' dendritic ' is usually applied would 

 have resulted. To me it seems impossible that one layer can be 

 fluid enough to be squeezed into another, and yet coherent enough 

 to move only in masses. 



Again, if pressure were the sole cause, why should not the dark 

 matter have been squeezed downwards below its point of origin as 

 well as upwards (4-9), and why were not the thinner horizontal 

 bands of dark matter also displaced (7, 9), or at least bent irregu- 

 larly, whilst the shrinkage was going on ? 



YII. IfoDE OE FORMATION OF THE LANDSCAPE MAEBLE. 



Perhaps it will be well to say here that I commenced my investi- 

 gation of Landscape Marble with the preconceived idea that its 

 peculiar characteristics were due to interbedded layers of vegetable 

 matter, which continued to decompose and evolve carbonic-acid gas 

 and marsh gas after its deposition ; and that where a layer of extra 

 thickness occurred the decomposition continued whilst a thickness of 

 several inches of new sediment was laid down, with the result that 

 arborescent markings were produced along the lines taken by the 

 escaping bubbles. The excessively wrinkled upper surface of the 

 stone seemed to necessitate a central portion capable of consider- 

 able shrinkage, and organic matter provided such a material. 



A careful examination of all the characteristics of the stone, and 

 some experiments aiming at its artificial reproduction, have con- 

 firmed the main idea, but have caused me to modify the expla- 

 nation of the formation of the crinkled upper surface. 



The rock ivas without doubt a sedimentary one (1, 4-9, 16, 17) ; 

 it occurs in the midst of sedimentary rocks (18), and is intimately 

 associated with a fairly persistent bed (16), although it might at 

 times appear to be merely a concretion (2). The flaking off some- 

 times observed is due to original difference of sediment (3), and to 

 the present different physical characters of the light and dark 

 matter respectively ; it occurs in the nearly horizontal lower layers 

 as well as in the upper undulating ones (10, 11, 12). 



It has undergone considerable change since its deposition, both 

 physical (1, 3-9) and chemical (10-13), at least so far as the re- 

 construction of some of the crystallizable matter and oxidation 

 of the outer layers in some specimens (7, 9) are borne in mind, 

 without as yet considering the changes in the organic matter con- 

 nected with the formation of the arborescent markings. 



The conchoidal fracture of the stone (1) is a characteristic usually 

 observed in fine-grained limestones, I believe, and does not, therefore, 

 need special treatment. 



1 One of the single stems, without bifurcation, I have found to be more than 

 | inch long, and from fa to § inch wide. 



