﻿Unconformity and Thrust in Coal-Measures. 171 



a series of years. The unconformity is shown by discordance in dip, 

 by overlap of the Table-Hocks Sandstone, and by the existence of a 

 pebble-bed, containing fragments of the mussel-band and other parts 

 of the underlying series, in the lower part of this sandstone. But 

 the upper, more massive, beds in the section have been thrust in a 

 northerly direction over the lower and more yielding beds, the 

 plane of gliding corresponding accurately along parts of the section 

 with the plane of erosion. Towards the north of the section the 

 beds of the upper series are weakened by intercalated bands of 

 shale, and then differential action has been set up. The result is 

 that the thrust-plane is no longer a simple one coinciding with the 

 unconformity, but extends some way above it. The effects of 

 the thrust are seen in the ploughing-up, folding, and faulting of the 

 lower series, in the penetration of tongues of sandstone from the upper 

 series into the lower, in the curling-up and shattering of the 

 pebble-bed, in the puckering and hardening of the shale, and in 

 the blending of fragments of the various rocks subjected to its 

 influence. The marks of intense action are practically confined to 

 the surface of the beds of the lower series. The action of the thrust 

 is markedly rhythmical, and its effect is to mask the unconformity ; 

 and the great discordance in dip which is at times produced is no 

 criterion of the unconformity. Although the amount of rock 

 removed by intro-Carboniferous denudation from the lower series 

 is unknown, chemical and other evidence is given to show that the 

 lower series has undergone weathering and some leaching-out of 

 constituents in the interval between the two series. 



2. 'The Carboniferous Succession below the Coal-Measures in 

 North Shropshire, Denbighshire, and Flintshire.' By Wheelton 

 Hind, M.D., B.S., F.R.C.S., F.G.S., and John T. Stobbs, F.G.S. 



April 25th.— J. E. Marr, Sc.D., F.R.S., Vice-President, 

 in the Chair. 



The following communications were read : — 



1. ' Trilobites from Bolivia, collected by Dr. J". W. Evans in 

 1901-1902.' By Philip Lake, M.A., F.G.S. 



2. ' Graptolites from Bolivia, collected by Dr. J. W. Evans in 

 1901-1902.' By Ethel M. R. Wood, D.Sc. 



3. ' The Phosphatic Chalks of Winterbourne and Boxford 

 (Berkshire).' By Harold J. Osborne White, E.G.S., and Llewellvn 

 Treacher, F.G.S. 



Data collected in the district dealt with in this paper suffice to 

 show that the more or less Phosphatic Chalks above the Uintacrinus- 

 band lie in a trough or basin, the formation of which antedates the 



