IN THE STRATA OF THE CARBONIFEROUS SERIES. 33 



means of accounting for the fact. For instance, large fossil trees are found 

 in the mountain limestone groups. In this series, we have abundance of 

 lime, and the reason why the sandstone contains less lime than the fossil, 

 appears to be, that before the strata were consolidated, the sand being more 

 incoherent than the wood, the lime found a more easy passage through it, 

 and was not therefore retained in so great quantity as in the fossil, the 

 vacuities left by the decay of which it gradually filled up. 



Dr Gregory, who has been kind enough to examine the Craigleith 

 fossil, has favoured me with the following remarks upon it. " The iron 

 exists in the state of peroxide in the mineral. If reduced to protoxide, it 

 would amount to 21.8 per cent., and the loss would be 2.4 per cent, of which 

 a part is water. The coaly matter is, like ordinary coal, partly volatile, 

 yielding inflammable gas and bituminous matter by heat. The residue is 

 nearly pure charcoal about 14.3 per cent. The mineral contains very mi- 

 nute traces of silica and alumina. The other fragments have a similar 

 composition, only the lime is more abundant, from the presence of some 

 crystals of calcareous spar, which it is impossible to separate completely." 



It will be seen afterwards, that the composition of the fossil stems of 

 the coal formation, differs essentially from that of the trunks occurring in 

 this series of rocks, some of the former having as much as 95 per cent, of 

 silica. 



Having described the external appearance of the Craigleith trunks, and 

 examined their transverse sections with the microscope, we now proceed to 

 inspect their longitudinal sections. 



In Plate VII., are represented thin slices as viewed by the microscope. 



Fig. 1. Represents a small portion of a slice cut longitudinally parallel 

 to the medullary rays, taken from the fossil tree of 1826'. The texture of 

 this stem was very much altered and distorted, so that it was difficult to 

 obtain characteristic specimens. In the minute portion represented, as well 

 as in several other specimens, there were seen distinct appearances of elon- 

 gated cellules similar to those of the Coniferae ; but, as is observed in the 

 figure, the walls of these cellules, instead of presenting a single series of se- 

 parated circular glands or pores, were marked with series of parallel longi- 



e 



