512 JR. Lydekher — On Sylceochampsa. 



the weight of the overlying rock had been removed, it rose in a 

 slight arch over the npthrust shale, and was thrown into a state of 

 tension, so that when struck, chips and flakes, and sometimes larger 

 pieces, would fly oif. These pieces were in themselves quite sound. 

 It was not that the whole mass was like Rupert's drops in a state of 

 molecular unstable equilibrium, as suggested by Mr. Adam, and 

 supported by Mr. Strahan in the case of the explosive slickensides, 

 but it was rather analogous to the effect of drawing a knife across 

 the outside curve of a bent stick ; when jagged ends spring off and 

 stand out straight in the original direction of the unbent stick — only 

 in this case the fibrous character of the material prevents the pieces 

 breaking away altogether as if it were rock. That the conditions in 

 the case of the explosive slickensides were similar to those at Dent 

 Head Quarry and the Kibble Head tunnel seems probable from the 

 observation of Watson that " this phenomenon has not been noticed 

 from slickensides whei-e no shale is incumbent," and also from the 

 suggestion of Phillips, " that the removal of one side of a vein would 

 leave the remaining side in a condition of strain resembling that of 

 a strung bow, with a tendency to bulge outwards into the workings. 

 The undercutting would free, so to speak, one end of the bow." 



But the observations at Dent Head Quarry and Kibble Head tunnel 

 would show that it was not merely a bringing down of the mass of 

 the vein stuff when undercut that was remarkable, but rather the 

 tendency to shell off in chips and flakes over the whole surface. 

 The beds at Dent Head and Kibble Head were approximately hori- 

 zontal. Also it would appear from analogy that what the vein stuff 

 did was not to offer a material more readily burst up, but a mass of 

 the form and extent which could be bent by settlement of the sur- 

 rounding rock so as to be thrown into a state of tension all over the 

 exposed surface. 



In connection with these inquiries we must always remember that 

 time is an element in the bending of rocks, and that it is the rapidity 

 of the action due to the artificial removal of the overlying mass that 

 causes the rock to break rather than to sag and retain its curved 

 form, as we see so commonly among the contorted strata where 

 nature has applied the pressure more gradually. 



IX. — Note on Htljeocrampsa. 

 ByE. Ltdekker, B.A., F.G.S. 



IN my paper on Crocodilia in the July Number of the Geological 

 Magazine, pp. 310-311, I expressed my opinion that the 

 characters given by M. Dollo in his description of the Belgian 

 Bernissartia did not appear to me to afford grounds by which that 

 form could be distinguished from Hyloeochampsa, and I accordingly 

 observed in a note that the onus of proving the distinctness of the 

 former rested with its desci'iber. In a note published on pp. 394- 

 396 of the September Number of the Magazine, M. Dollo replies 

 to this criticism, and shows conclusively that the Belgian fn-in is 

 entitled to specific, and very probably to generic distinction from the 

 larger English one. 



