CiRC. 154. 



often remarkably so in certain portions. The interior bedding, where such exists, 

 is horizontal or follows the dip of the basement rocks, but the external bedding is 

 usually quaquaversal. Around their base and lower sides occurs a breccia of lime- 

 stone fragments in either a limestone matrix or a matrix of the overlying shales. 



Mr. Tiddeman was the first to suggest the origin of these very puzzling hills, 

 and he thinks they have been built up by marine organisms in a slowly-sinking 

 area, very much as coral reefs have. He considers that the nature of the rocks, 

 the abundance of fossil contents, the dip of the bedding, and the breccias at the 

 foot, support this hypothesis. 



Mr. J. E. Marr offers a different explanation. He regards them as due to 

 orogenic movements, and says : — ' Between the comparatively rigid Millstone 

 Grits above and the Lower Palaeozoic Rocks below, lie the more yielding group 

 of shales and limestones above these. The formation of the folds with north- 

 easterly and south-westerly axes must have resulted in the accumulation of the 

 rigid Millstone Grits above in great masses into the synclines, and their dragging 

 away from the tops of the anticlines. There may have been actual rupture of the 

 grits of the arches, but in any case there would be relief of pressure there, causing 

 the limestones to be squeezed out frorh below the synclines and to accumulate in 

 the anticlines, and I believe that it is in this way the knoll-reefs were formed, 

 while the minor faults, which would inevitably accompany repetition of the strata, 

 are marked by breccias differing in character according to the nature of the rocks 

 in which they were formed. If my views be true, we are dealing with a great 

 group of yielding strata, sandwiched in between two more rigid rock-groups, and 

 all affected by the thrusting of other strata over them from the north.' Mr. Marr 

 thinks that the relief from pressure in the anticlinals may account for the preserva- 

 tion of the fossils in the Knolls. 



References : — Tiddeman, Brit. Assoc. Report, 1889 ; Tiddeman, Brit. Assoc. 

 Handbook, 1890, Leeds Meeting; Marr, Quart. Journ. Geol. Soc, vol. 55, no. 

 219 ; Dakyns, ditto. 



