Yol. 50.] MR. B. THOMPSON ON LANDSCAPE MARBLE. 403 



the other evidence as to an organic origin for the dark layers and 

 marks. 



The mammillated or corrugated upper surface of Landscape Marble 

 (3) is so distinctive that it must evidently require an explanation 

 dependent upon the other peculiar characteristics ; ordinary desicca- 

 tion is quite inadequate, otherwise we should much more often 

 meet with similar characteristics in other stones. At first, I thought 

 that the excessive wrinkling of the upper surface of the stone was 

 due to exceptional contraction of the interior, and this an interior 

 composed partly of vegetable matter would give ; but I was quite 

 conscious that this was not all the truth, because it did not suffici- 

 ently account for the wrinkling occurring only on the upper surface. 



Since I commenced writing this paper I quite accidentally obtained 

 a very near approach to the appearance of the 'rustick' on Landscape 

 Marble, thus : — Some zinc dust was mixed with copper sulphate 

 (Zn.Cu. couple) ; after some time the compound produced was 

 washed and left suspended in distilled water — I say suspended, for, 

 although prepared more than a month ago, it has never really settled, 

 but constantly occupies much more room than it would do as a dry 

 solid, leaving some water at the top quite clear. This preparation 

 has slowly, but continuously, evolved hydrogen, but to disengage 

 the bubbles, which are below the surface of the suspended matter, 

 a slight shake is necessary. The particular point, however, is this, 

 that whenever the vessel has been left quiet for a day or two the 

 upper surface of the solid is covered with hills and hollows greatly 

 resembling the upper surface of Landscape Marble. When the flask 

 is shaken, bubbles of gas escape, the hills collapse, and gradually 

 an even surface is produced again. The bubbles of gas do not 

 generally escape from the hillocks, but rather from between them ; 

 nevertheless, when the gas has escaped the contiguous hillocks fall, 

 showing that the elevation of the surface of the solid was due to the 

 upward pressure of the hydrogen. 



I consider, therefore, that the upper corrugated surface of the 

 Gotham Stone is chiefly due to the upward pressure of the same gases 

 that produced the arborescent markings, after their escape had been 

 prevented by increasing coherence or greater thickness of the upper 

 layers of sediment. 



The fact that the arborescent markings rise where there is a 

 hillock, and that they are absent altogether under the larger 

 depressions, at least in my specimens (see fig. 1, p. 395), renders the 

 above explanation all the more plausible, though shrinkage must 

 also be given its just due (9). 



The curvature of some specimens of Gotham Stone is evidently a 

 later production than the stone itself, for it affects all the layers 

 alike (6), and so need not detain us much. Since the curvature is 

 sometimes very irregular (see fig. 2, p. 397), it seems that it is most 

 likely due to earth-movements. 



The physical constitution of the Coiham Stone as revealed by the 

 microscope does not permit, perhaps, of so certain an explanation as 



