LAUEENTIAN AND EAELT PALAEOZOIC 35 



by Fleming and Carruthers * to be casts of caTities filled 

 ■with fluid, abound in the shales of the Carboniferous and 

 Devonian. They are, no doubt, in most oases the results 

 of the pressure and consolidation of the clay around smaU 

 solid bodies, whether organic, fragmentary, or concre- 

 tionary. They are, in short, local slickensides precisely 

 similar to those found so plentifully in the coal under- 

 days, and which, as I haye elsewhere f 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 appearances of this kind in connection 

 with small stems, petioles, fragments of wood, and car- 

 polites. I have in my collection petioles of ferns and 

 fruits of the genus Trigonocarpum partially slickensided 

 in this way, and which if wholly covered by this kind of 

 marking could scarcely have been recognised. I have 

 figured bodies of this kind in my report on the Devonian 

 and Upper Silurian plants of Canada, believing them, 

 owing to their carbonaceous covering, to be probably 

 slickensided 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 slickensiding 

 is quite incompatible with a mere "fluid-cavity," even 

 supposing this to have existed. 



Prof. Marsh has well explained another phase of the 

 influence of hard bodies in producing partial slickensides, 

 in his paper on StyloKtes, read before the American As- 

 sociation in 1867, and the application of the combined 

 forces of concretionary action and slickensiding to the 

 production of the cone-in-cone concretions, which occur 

 in the coal-formation and as low as the Primordial. I 

 have figured a very perfect and beautiful form of this 



* "Journal of the Geological Society," June, 18'?1. 

 f Ihid., vol. X., p. 14. 



