58 J. 8. O. Wilson 8^ H. B. Muff— The Hill of Beath. 



dip close to the tuff was also proved in the ' levels ' in the Five Foot 

 coal on both sides of the roadway. At the actual contact the shales 

 are much crushed, dragged out, and slickensided, and the sandstones 

 are shattered, but there is no sign of contact alteration, which some- 

 times occurs at the edge of other necks. On the western side the 

 stone-mine has penetrated a massive white sandstone, which shows 

 no signs of bedding, so that it is not yet possible to say whether the 

 usual downward flexure of the beds takes place here also. 



Further evidence of the behaviour of the tuff is afforded by the 

 now closed down Halbeath Colliery, in which the coals were worked 

 close up to the south wall of the neck, imtil they began to dip down 

 towards it at an angle of 24°. At the south-east corner of the hill the 

 coals were worked for a short distance vertically beneath the surface 

 crop of the tuff, thus showing that the wall of the neck was here not 

 vertical but inclined inwards. The angle of inclination, however, is 

 not large. Thus the neck has not only been pierced from side to 

 side and found to consist of volcanic tuff, but it is known to possess 

 approximately vertical walls around more than half its periphery. 



The cause of the characteristic downward flexure of the strata 

 surrounding necks is not perfectly clear. The deformation must 

 often have taken place very slowly, for massive beds of sandstone, 

 etc., are in many cases bent without being broken, though in other 

 instances the beds are fractured and displaced. Slickensides are 

 frequently found on the walls of necks, and the clearest exposures 

 closely resemble sections of a fault in which the strata on the upthrow 

 side have been dragged downwards and the weaker beds crushed and 

 drawn out. In the case of a neck the fault must be a circular one, 

 the whole mass of material filling the neck having subsided bodily. 

 If the subsidence were due merely to the consolidation and consequent 

 contraction of the infilling volcanic rock, the amount of the flexure 

 should decrease with the depth. This is not corroborated by the 

 examination of neck- junctions underground. Sir Archibald Geikie 

 points out that "after copious eruptions, large cavernous spaces may 

 conceivably be left at the roots of volcanoes, and the materials that 

 have filled the vents, losing support underneath, will tend to gravitate 

 downwards, and if firmly welded to their surrounding walls may drag 

 these irregularly down with them ".^ The phenomena at the margin 

 of the Hill of Beath neck are very much what might be expected if 

 such a subsidence had taken place. Though the surrounding sediments 

 are dragged downwards, the tuff shows no signs of disturbance except 

 near the margin, where it is cracked and the fissures occupied by 

 calcite. 



The material filling the neck is a palagonite tuff quite uniform in 

 character throughout. It is composed of greenish vesicular lapilli 

 scattered through a dull bluish-grey matrix, in which sand-grains can 

 be made out. The lapilli vary in size from particles only just visible 

 to the naked eye to fragments an inch or more across. The tuff is not 

 stratified, strictly speaking, but it exhibits in the mine a broad striping 

 due to bands containing large lapilli alternating with others made up 

 only of smaller ones. 



^ Loc. cit. , vol. i, p. 73. 



