550 PROF. G. A. LEBOUR AND DR. J. A. SMTTHE ON [Aug. I906, 



there is often no noticeable discordance of dip between the two 

 Series, and the pebbles form, in snch cases about the only means of 

 detecting the unconformity. The stormy episodes are generally 

 marked by the elevation above the general level of the eroded 

 surface of the lower shales. Thus, to some extent, the thrust 

 masks the unconformity, since the wavy uneven outline of the 

 surface of the Lower Series is partly the result of the disturbances 

 caused by the thrust ; and, furthermore, violent discordance of dip 

 between the upper and the lower beds, in so far as produced by 

 thrusting, is no criterion of the unconformity. 



EXPLANATION OF PLATE XLIL 



Section from M to N (see the sketch-map, fig. 1, p. 532), on the scale of about 

 18 feet to the inch, vertical and horizontal. 



Discussion. 



The Chairman (Mr. R. S. Herries) said that the paper once more 

 showed the value of a series of observations conducted over a long 

 period in a particular district. 



Mr. Lampltjgh complimented the Authors on their clear exposition 

 of this striking example of disruption between 'strong' and 'weak' 

 beds. He was reminded of analogous phenomena in another East- 

 Coast section, on the north side of Flamborough Head, where the 

 massive flinty Chalk had been locally thrust over the more thinly - 

 bedded Lower Chalk. He would ask the Authors whether they 

 had considered the possibility that the supposed pebbles might be 

 due to the breaking-up of the lower beds during the movement. 



Mr. Fox Strangways said that the previous speaker had referred 

 to the thrust-plane which occurs at the junction of the flinty Chalk 

 with the non-flinty Chalk in the cliffs of Flamborough Head. As 

 he (the speaker) knew this section very well, he was happy to 

 endorse Mr. Lamplugh's remarks, and to state that the rocks at the 

 foot of Buckton Cliffs appeared to have been subjected to very much 

 the same conditions as those mentioned by the Authors at the base 

 of the Table-Rocks Sandstone ; also that the peculiarities noted by 

 them were, no doubt, due to the thrusting of this rock over the 

 beds below. 



Prof. Watts pointed out that the paper was of considerable 

 interest, because it showed the result of years of denudation on the 

 thrust-plane, which was a common phenomenon at the junction of 

 strong and weak rocks. The Authors seemed to have established 

 the fact of unconformity, and that thrust was insufficient to explain 

 the whole of the phenomena. 



Mr. R. H. Tiddeman, without offering an opinion on the suggested 

 unconformity, said that frequently, where Carboniferous sandstone 

 rested on clay Or shale, the upper bed contained pebbles of the 

 lower ; but such erosion did not necessarily imply any considerable 

 unconformity. 



