34 



THE GEOLOGICAL HISTORY OF PLANTS. 



and which sometimes communicates to them a marvellous 

 resemblance to the netted under surface of an exogenous 

 leaf. Flattened stems of plants and layers of cortical 

 matter, when carbonised, shrink in such a manner as to 

 produce minute reticulated cracks. These become filled 

 with mineral matter before the coaly substance has been 

 completely consolidated. A further compression occurs, 

 causing the coaly substance to collapse, leaving the little 

 veins of harder mineral matter projecting. These im- 

 press their form upon the clay or shale above and below, 

 and thus when the mass is broken open we have a car- 

 bonaceous film or thin layer covered with a network of 



raised lines, and 

 corresponding mi- 

 nute depressed 

 lines on the shale 

 in contact with it. 

 The reticulations 

 are generally ir- 

 regular, but some- 

 times they very 

 closely resemble 

 the veins of a re- 

 ticulately veined 

 leaf. One of the 

 most curious speci- 

 mens in my pos- 

 session was collect- 

 ed by Mr. Elder 

 in the Lower Car- 

 boniferous of Horton Bluff. The little veins which form 

 the projecting network are in this case white calcite ; but 

 at the surface their projecting edges are blackened with 

 a carbonaceous film. 



Slichensided bodies, resembling the fossil fruits de- 

 scribed by Geinitz as Gulielmites, and the objects believed 



Ym. 11. — Cast of shrinkage cracks (Carbon- 

 iferous, Nova Scotia), illustrating pre- 

 tended Akfse. 



