No. 2.] DAWSON — IMPRESSIONS AND FOOTPRINTS. 73 



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 pro- 

 jecting. These impress their form upon the clay or shale above 

 and below, ajid thus when the mass is broken open we have a 

 ■carbonaceous film or thin layer covered with a network of raised 

 lines, and corresponding minute depressed lines on the shale in 

 'Contact with it. The reticulations are generally irregular, but 

 -sometimes they very closely resemble the veins of a reticulately 

 veined leaf. One of the most curious specimens in my posses- 

 sion was collected by Mr. Elder in the Lower Carboniferous of 

 Horton BluflF. The little veins which form the projecting net- 

 work are in this case white calcite ; but at the surface their pro- 

 jecting edges are blackened with a carbonaceous film. 



Slicken-sided bodies, resembling the fossil fruits described by 

 Oeinitz as Gulielmites, and the objects believed by Fleming and 

 Oarruthers* to be casts of cavities filled with fluid, abound in 

 the shales of the Carboniferous and Devonian. They are, no 

 •doubt, in most cases the results of the pressure and consolidation 

 •of the clay around small solid bodies, whether organic, fragmen- 

 tary or concretionary. They are, in short, local slicken-sides 

 precisely similar to those found so plentifully in the coal under- 

 •clays, and which, as I have elsewheref shown, resulted from the 

 internal giving way and slipping of the mass as the roots of 

 •Stigmaria decayed within it. Most collectors of fossil plants in 

 the older formations must, I presume, be familiar with appear- 

 ances of this kind in connection with small stems, petioles, 

 fragments of wood, and carpolites. I have in my collection 

 petioles of ferns and fruits of the genus Trigonocarpum partially 

 slicken-sided in this way, and which if wholly covered by this 

 kind of marking could scarcely have been recognized. I have 

 figured bodies of this kind in figs. 126 and 231 of my report on 

 the Devonian and Upper Silurian plants, believing them, owing 

 to their carbonaceous covering, to be probably slicken-sided 

 fruits, though of uncertain nature. In every case I think these 

 bodies must have had a solid nucleus of some sort, as the severe 

 pressure implied in slicken-sliding is quite incompatible with a 

 mere " fluid-cavity," even supposing this to have existed. 



•Journal of Geol. Society, .June, 1871. f Ibid, vol. x, p. 14. 



