. Geological magazine. slickensides, reminded me ofsome similar phenomena which were not of unfrequent occurrencenear Dent Head and Kibble Head in Yorkshire. In the limestone quarry from which the black marble of Dent isprocured the workmen found that, when they were quarrying thelower beds and struck the rock with a pick or bar, fragments flewup into the air with greater force than could be due to their blowand in an unexpected direction. Also, when the tunnel was being made above Eibble Head, andthe workmen were engaged upon the bed of rock which formed thefloor of the tunnel, pieces used to
. Geological magazine. slickensides, reminded me ofsome similar phenomena which were not of unfrequent occurrencenear Dent Head and Kibble Head in Yorkshire. In the limestone quarry from which the black marble of Dent isprocured the workmen found that, when they were quarrying thelower beds and struck the rock with a pick or bar, fragments flewup into the air with greater force than could be due to their blowand in an unexpected direction. Also, when the tunnel was being made above Eibble Head, andthe workmen were engaged upon the bed of rock which formed thefloor of the tunnel, pieces used to burst off with a loud noise, sothat some thought they had discovered a detonating shale. The explanation in both these cases seemed to be that the bedwhich was apt to shell off in that unexpected manner rested on shalewhich yielded to the superincumbent weight on either side, andproduced in the tunnel, or in the quarry, where the overlying rockhad been removed, what would be called in a coal-mine a creep(see Woodcut).. Diagram showing the manner of occurrence of Bursting rock at Dent Head andKibble Head in Yorkshire, a rock, b shale ; the arrows denote the direction ofthe pressure. The shale behaves as a thick fluid or viscous mass, and transmitsthe pressure and motion. But in the cases to which I refer, a thinbed of the solid rock was left above the shale. This was not com-pressible, but, where, in the tunnel or in the centre of the quarry, 512 JR. Lydekher—On Sylceochampsa. the weight of the overlying rock had been removed, it rose in aslight arch over the npthrust shale, and was thrown into a state oftension, so that when struck, chips and flakes, and sometimes largerpieces, would fly oif. These pieces were in themselves quite was not that the whole mass was like Ruperts drops in a state ofmolecular unstable equilibrium, as suggested by Mr. Adam, andsupported by Mr. Strahan in the case of the explosive slickensides,but it was rather analogous to the effect of dra
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