400 ME. B. THOMPSON ON LANDSCAPE MAKBLE. [Aug. 1 894, 



as might be expected, there is evidence also of chemical change. 

 In attributing the corrugated surfaces to the shrinking of the 

 calcareous mud, I may have appealed too strongly to mechanical 

 causes, as apart from the obscure processes of segregation, or even 

 of concretionary action." 



A perfect explanation of the origin of the Landscape Marble 

 should, of course, account for all the characteristics enumerated 

 (1 to 17) ; but if this be impossible we must remain content with one 

 that will account for the greater number, without being obviously at 

 variance with any. Let us first of all clear the ground of those 

 explanations or theories, or parts of theories, which are at variance 

 with the known characteristics of the stone, so that the explanation 

 which I shall presently offer may be less encumbered. 



The escape of imprisoned air. — Edward Owen's theory cannot be 

 entertained as an explanation, even if we give ' air ' the wider 

 signification of ' gas,' which it may have had to Owen in 1754, for 

 gas could not have been imprisoned in a fine soft mud such as this 

 stone must have been when forming ; nevertheless, after reading 

 Owen's description, I am inclined to think that he had a clearer 

 conception of the method of formation than he was able, through 

 lack of chemical knowledge, to express. 



Gaseous emanations from the black mud of the Avicula contorta- 

 shcdes (18) must also be discarded as a theory, for the simple reason 

 that in most specimens, perhaps in all, the lower portion of the stone 

 (the part that rested on these shales) is quite free from the black 

 arborescent markings, and in place of them are horizontal ones 

 alternating with other sediment (7). The not uncommon presence 

 of two landscapes (5) is also quite inexplicable by this theory. 



Infiltration of dark mineral matter cannot be accepted as a cause 

 of the arborescent markings, because they spread out upwards and 

 not downwards, and because they are bounded, both above and 

 below, by the same dark matter in regular bands lying in an 

 approximately horizontal position (4-9), and also because the mark- 

 ings are much too large, and much too distinctly and evenly bounded 

 (11). For the same reasons, and others, mere staining is quite out 

 of the question. 



Shrinkage of the stone, whether before or after consolidation, 

 cannot alone be admitted as a cause of the arborescent markings, 

 though for other reasons we must admit considerable shrinkage. 



Shrinkage after consolidation could not have caused it, because 

 then there would be an absence of fluid or semi-fluid matter such as 

 must be postulated to account for one kind of mineral matter 

 becoming so irregularly mixed with another. 



That shrinkage before or during consolidation cannot be admitted 

 as a cause is equally certain, because in such a fine-grained rock as 

 this (1, 10) the interspaces between the particles (or crystals, if 

 they then existed) of the matrix would be much too small to admit 

 of the intrusion of such comparatively large masses of foreign 



