28 Reviews — Geological Survey of Great Britain. 



the south-eastern margin of the coal-field, by conformable overlap. 

 The Trenchard Coal and the underlying Trenchard Measures are 

 overlapped by the Pennant Sandstone above them. As a result, 

 the Pennant comes to rest directly and uncouformably upon the 

 older, steeply inclined strata, and the Trenchard Measures fail to 

 crop over a considerable part of the distance between Staple Edge 

 Wood and the Cannop Yalley. This is abundantly clear at Danby 

 Lodge, where the Pennant Sandstone, containing the Coleford High 

 Delf Coal, transgresses the quartz-conglomerates of the Old Red. 

 It is confirmed by the section in an old quarry on the northern side 

 of the Blackpool Valley, which shows the unconformable junction of 

 Pennant Sandstone and Carboniferous Limestone. The sandstones 

 in this quarry dip gently westwards. They show le.nticles of clay 

 and a streak of very coarse grit or quartz-conglomerate at their base, 

 and repose upon the worn, hummocky edges of dolomite-beds which 

 dip north-westwards at about 60°. 



To sum up, an unconformity at the base of the Coal Measures is 

 a dominant feature in the geological structure of the Forest of Dean. 

 It is evidenced (1) by the overstep of the Coal Measures across the 

 Drybrook Sandstone, the Carboniferous Limestone, and the Old Eed 

 Sandstone, (2) by a great difference between the prevailing dip of 

 the older strata and that of the Coal Measures along the eastern 

 margin of the coal-field, and (3) by visible discordance of dip at 

 exposed junctions of the Coal Measures with Drybrook Sandstone and 

 Carboniferous Limestone respectively. It is attended by (1) the 

 development of basal conglomerates in the Coal Measures, pnrticularly 

 well seen on the northern border of the coal-field, and (2) local 

 overlap of the Trenchard Measures by the Pennant Sandstone, 

 whereby the former are concealed along part of the south-eastern 

 edge of the coal-field. 



DaEA7'iEi'^;?^s. 



I. — Geological Stjkvey of Great BRrrAiisr. 

 Summary of Peogrkss of the Geological Survey of Great Britain^ 

 FOR1916. 8vo; pp. iv-|- 56 and 3 figures in text. London, 1917. 

 Price Is. 6d. 



\S was only to be expected, the energies of the Geological Survey 

 have been almost entirely diverted into new channels connected 

 with the War. Ordinary field work and detailed mapping are 

 completely suspended, and the remaining staff has devoted itself to 

 the investigation of certain pressing problems connected with the 

 mineral resources of the country. Almost the only exception to this 

 statement is the continuance of the work of examining bore-holes 

 now in pi'ogress ; this information, if not recorded at once, is 

 necessarily most difficult to recover at a later date. Five volumes of 

 Special Reports have been published dealing with the occurrence of 

 certain minerals of economic value, and a further volume is in 

 preparation on the subject of refractories; these include sandstones, 

 quartzites, ganister, sands, and fireclays (acid refractories), as well as 

 the basic rock dolomite. These are used for furnace linings and 



