8 INTRODUCTION. 



lignified ribs projecting on the surface above the shrunken parenchyma. 

 Williamson * has explained this peculiar state of preservation. As the 

 rind of the Sigillarias went by the name of Dictyoxylon, Will., when its 

 connection with the stem was not yet recognised, the word, though now 

 superfluous, may still be used to designate this particular form of structure. 

 There are many species from the Carboniferous which had these Dicty- 

 oxylon-rinds. 



Other plants beside the Sigillariae, the Lepidodendrae for example, occur 

 in the form of impressions which represent the different surfaces of denuda- 

 tion of the stems. In transverse sections through the casts of Stigmarias 

 we often observe a narrow circular line of fissure which appears to be filled 

 with traces of coal. Longitudinal fracture shows that this line answers to 

 the bounding surface of a second exactly cylindrical cast lying within the 

 other, and having its surface marked by entirely different and characteristic 

 features of its own. There can be no doubt that this inner cast owes its origin 

 to the circumstance, that on the destruction of the inner tissue previously to 

 the filling in with mineral matter a hollow cylinder of tissue answering to 

 the inner side of the secondary wood resisted decay longer than the rest, 

 so that when it disappeared two hollow cylinders of preserved tissue remained 

 lying one inside the other. The inner cast answers to the inner space 

 occupied by the inner tissue, the outer to that occupied by the outer. The 

 dividing layer cannot indeed have been very substantial, for in that case it 

 must have appeared in the form of an evident rind of coal. 



A quite peculiar and usually rare kind of mould and formation of a cast 

 has recently played an important part in the controversy which has arisen 

 between Gaston de Saporta 2 and Nathorst ;i respecting the algal nature of 

 certain fossils. In some regularly stratified deposits (Saporta gives as his 

 chief instance the Kimmcridge strata of Cirin with branches of coniferous 

 plants) on the under surface of the beds casts are found, which project in 

 half-relief only, and fill corresponding depressions forming half-moulds in 

 the bed as it lies. There is no rind of coal ; mould and cast show the same 

 sculpture negative and positive. There are only two ways of explaining 

 such cases as these. They may have arisen from the investment of parts 

 of plants which contained little solid material and much water, and which 

 soon collapsed, such as are of frequent occurrence among Algae. The thick 

 algal thallus collapsed immediately after investment, the mud which covered 

 it being still soft sank with it, and the organic substance was so scanty that 

 it oozed away in the under-layer, or at any rate could not give rise to any 

 appreciable quantity of coal. But this mode of explanation does not suit 

 branches of Conifers, and in their case we can only imagine, that after being 

 deposited in the mud they were then removed from their beds by subse- 



1 \Yilliamson (1), iv. 2 de Saporta ^1). * Nathorst (1). 



