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



the macroscopic characters, but, in conjunction with the chemical 

 composition, it forms a study of much interest. 



The fine granular calcite (10) may have been an actual deposit of 

 detrital matter from adjacent calcareous lands, or a chemical pre- 

 cipitate from warm shallow waters. I incline to the latter belief 

 for these reasons : («) the limestone is fairly pure ; (b) the carrying 

 power of water which deposited such frequent alternate and very 

 thin beds of vegetable matter must have been insignificant. 



The presence of small grains of quartz (10, 15) does not consti- 

 tute any great obstacle to this idea, because they, together with 

 the argillaceous matter, may have been brought with the vegetable 

 matter. 



The formation of the coralloidal aragonite, the presence of which 

 in one of my specimens so accurately coincides with the limits of 

 the arborescent markings and dark bands, is a problem of much 

 interest, and seems to point to organic matter as being sometimes 

 a determinative agent in the crystallization of calcium carbonate. 



The superior lustre of the arborescent markings and dark bands is 

 most obvious if a polished specimen of the stone be held before a 

 window, so that the light falls upon it very obliquely ; indeed, they 

 have then quite a metallic appearance, so much so that at first I 

 looked for a sulphide of some metal. In one specimen the higher 

 refractive power of aragonite accounts for its higher reflective power 

 than the calcite matrix, but its greater compactness or non-granular 

 texture may materially contribute to that result. In another 

 specimen other causes must be sought (see below). 



The darker colour of the nearly transparent aragonite might be 

 due to its greater transparency if surrounded by opaque matter. 

 This assumption, however, is negatived by the fact that the 

 aragonite is really darkest where thinnest, that is, at the outer 

 boundary (11). The darkness is therefore due to contained darken- 

 ing matter — carbon. Quite apart from the fact that it is difficult 

 to account for the even dissemination of a heavy substance like 

 oxide of manganese in a vertical direction, the amount of manganese 

 indicated by a qualitative analysis seemed rather too small to fit 

 the explanation. 



In the specimen of stone (fig. 2, p. 397), where the presence of 

 aragonite in the dark markings is not obvious (12), the markings are 

 quite as lustrous, or even more so ; this may be due to a darker 

 background of carbonaceous matter (12), to the finer polish which 

 the finer granules give, or to the actual presence of some aragonite ; 

 possibly to all three causes, more or less. 



Whether aragonite was imperfectly formed in the first instance, 

 or has since been more or less degraded to calcite, seems a rather 

 unprofitable speculation, as the evidence appears equally favourable 

 to both theories ; the excessive amount of unoxidized carbon pointing 

 to the former, and the greater instability of aragonite pointing 

 rather to the latter explanation. 



