86 NOTES ON A FOSSIL WOOD FROM COX S BIGHT. 



but on grinding to fine powder, and using the specific gravity- 

 bottle method, it is considerably higher. This difference 

 seems to be due to the physical structure, and not to the 

 chemical composition, which shows the upper half to be 

 ahiiost pure iron pyrites (Fe. S.g), and can no doubt be 

 accounted for by the slightly porous nature of the mass, 

 resulting from the original wood tissue being entirely replaced 

 by sulphide of irou. The specific gravity of the lower portion 

 is also slightly low, due to the inclusion of quartz particles. 



The results of two analyses show the upper half to be 

 almost pure Fe. S.,, with traces of Si. O.o, metals of the copper 

 group, and, perhaps, some carbon, which latter were not 

 determined, owing to iusnlficient material to operate on. The 

 lower half gave 26 per cent. Si. O.o and 73 per cent. Fe. S.g,. 

 with traces of the copper group. No assay of gold and silver- 

 was made on account of the smallness of the specimen, though 

 it is probable that traces of these metals would be found. 



The microscopic examination of this specimen is most 

 interesting and instructive, and shows very clearly the exact 

 nature and derivation, and throws light on the striking 

 physical differences between the upper and lower portions. 

 Using reflected light and a two-inch objective, the fibrous 

 natxire of the upper portion is clearly seen, and on the longi- 

 tudinal surface these fibres, replaced by iron pyrites, appear 

 closely packed together, and retaining i-emarkably well their 

 original structure. The iron pyrites on the outer portion 

 shows little signs of crystallisation, but towards the centre of 

 the upper portion becomes more dense and semi-crystalline. 

 On a freshly fractured cross section the structuie of the 

 upper half is seen in an even more striking manner than in 

 longitudinal section. The ends of the fibres give circular 

 cross sections and appear as a mass of very minute little 

 rings closely packed together. These longitudinal and cross 

 sections show conclusively the origin of the specimen, though 

 I am unable to give the exact genus and species of the original 

 wood, but perhaps some of the botanically-inclined members 

 of the Society may be able to clear up this point. The 

 lower portion exhibits none of these peculiarities either in 

 longitudinal or cross section, and merely presents the ordinary 

 appearance of massive iron pyrites with embedded grains of 

 quartz of a subangular form, and is apparently purely a 

 mineral deposit, though the adjacent organic matter probably 

 had some part to play in its deposition from solution. 



Appended are three water-colour drawings, ISTo. 1 being that 

 of the specimen as seen by the naked eye, a portion having 

 been broken oflf for analysis; No. 2 a longitudinal view under 

 reflected light with a two-inch objective ; and No. 3 a cross 

 section under the same power and light, both of the upper 

 portion. 



