C— GEOLOGY. 63 



Care has to be taken in such an inquiry to ehminate a soui'ce of error 

 not hitherto taken into account, namely, the relative compressibility 

 of different sedimentary materials. Freshly deposited mud may contain 

 90 per cent, by volume of water, and even when reduced by time and 

 pressure to the condition of shale may still have 20 per cent, of inter- 

 space ; a bed of fairly consistent clayey mud might be reduced to one- 

 half its thickness. Sand, however, suffers scarcely any loss of bulk 

 once it has got past the condition of a quicksand. This source of error 

 is eliminated in the calculations relating to the split of the Silkstone 

 and Barnsley seams, and it is seen that the increase of thickness in the 

 sagged area far exceeds the total thickness of the sandstone present, so 

 that the sag is a real one and not the effect of the relative compressibility 

 of the measures. There may be cases in which there is no further 

 shore to the sag, and the seam once lost is lost for good and all. Such 

 might be the margin of a deltaic flat undergoing intermittent 

 depression. 



It has occurred to me to consider whether the sediments with which 

 the Staffordshire Thick Coal is subdivided need necessarily have de- 

 manded an earth movement to an extent corresponding to their aggregate 

 thickness; in other words, whether the aggregate thickness of the sedi- 

 ments plus the seams that they now separate were, in the uncompressed 

 original condition, materially different in thickness from the great un- 

 divided seam. I have not the data upon which to found an opinion, 

 but we are promised a full discussion of this seam, when I hope the 

 problem will receive attention. The idea I had in mind has apparently 

 been current for some time, for I find Mr. Walton Brown expressed 

 the opinion many years ago that the Coal Measures might be regarded 

 actually as a single coal-seam, with the necessary implication that the 

 sedimentary measures are in the natm'© of local interruptions. Some 

 measure of the reduction of thickness which the original substance has 

 undergone and some consequences will be considered later. 



I now turn to a fonn of split seam of extraordinary interest, which 

 has received comparatively little attention from geologists though 

 mining engineers must surely have a special comminatory formula to 

 express their sentiments thereon. The first example that came under 

 my notice was encountered in the eastern workings of the Middleton 

 Main Seam, at Whitwood Colliery, near Wakefield. Thin intercalations 

 of shale and other sedimentary materials, appearing at different horizons 

 in the seam, were found to thicken gradually to the east concurrently 

 with the gradual dwindling of the lower part of the seam. An explora- 

 tion was then carried out. The bottom coal was followed, but it was 

 found that though the underclay continued the coal disappeared, and 

 was wholly lost for a short distance when it reappeared. The top coal 

 rose over a steadily thickening shale parting, and disappeared into the 

 roof of the workings, but boreholes pi'oved that it was present above 

 a parting which was, at the maximum, 29 feet thick. At the farther 

 end of the heading the top coal came down and the integrity of the 

 seam was restored. Two other transverse explorations have proved the 

 same general arrangement on the same scale of magnitude and one 

 or both margins have been traced for a long distance, enabling the 

 1922 G 



