740 REPORT -1900. 



the synclinals can be tracedwithsufHcient approximation by drawing lines at right 

 angles to the dips and describing circles from the point of intersection of these 

 lines as a centre. The thickness of the beds between any two points is given by 

 the length of the radius intercepted by the two corresponding circles. To test this 

 method it has been applied to cases in which the course of the synclinals was 

 known : one of these was atibrded by the section across the northern side of the 

 South Wales coal basin, along the course of the river Sawdde, a distance of four 

 miles; the other by the section exposed in the cuttings along the Rhymney railway 

 through the Old Red Sandstone, north of Cardiff, nearly two miles in length ; in 

 both cases the results very closely approached the facts. 



Two points at which the dips are known, such as those of Ware andCheshuut, 

 are sufficient to determine one side of a synclinal. From the construction obtained 

 a basin is indicated having its axis running east and west and situated below 

 Enfield Lock on the river Lea. Its northern half is thirteen miles in breadth, the 

 thickness of the contained strata 29,500 feet: of this 19,000 feet are Silurian 

 and Devonian ; the remaining 7,500 feet are Devonian and Carboniferous. How 

 much of this upper portion is Devonian is unknown, no great thickness probably, 

 when the great thickness, 19,000 feet, of the underlying Devonian and Silurian 

 is considered. How much of the Carboniferous consists of productive Coal- 

 measures is also uncertain, but that there is ample room for an important coalfield 

 is shown by comparison with the Forest of Dean: that is only eight miles in width, 

 and the total thickness of the Carboniferous beds, upper and lower, contained by 

 it is only 3,500 feet, while the Enfield trough is approximately fourteen miles in 

 width and 7,500 feet in depth. An attempt to apply another method of deter- 

 mining the course of folds, employed by Professor Lapworth, shows that the 

 thickness of the Enfield measures may be even greater than here given — perhaps 

 10,000 feet. 



It is scarcely necessary to point out that faults and other troubles may exist . 

 by which these estimates may be modified, but the dip of the Devonian beds to 

 the south, as determined b}' the deep boring made at Meux's Brewery, London, 

 affords a strong confirmation of their general truth. 



The strike of the beds would suggest a locality somewhat west of Enfield Lock — 

 possibly near the town of Enfield or JS'ew Barnet — as the most promising spot for 

 a trial boring. 



2. On the Formation of Heef Knolls. Bij R. H. Tiddemax, M.A., F.G.S. 



[Communicated with the permission of the Director-General of the 

 Geological Survey.] 



At the meeting of the British Association at Newcastle iu 1889 I brought out 

 my interpretation of the probable origin of the limestone knolls of Yorkshire.^ 



It was shown that the Lower Carboniferous Rocks in the North of England 

 had two distinct types— that the Yoredale or Northern type extended from the 

 Craven Faults to the Tyne, and that the Southern or Bowiand type occupied the 

 country from the Craven Faults to near the Western Seaside plain and extended 

 south as far as Derbyshire. Without now recalling the two tables of the succes- 

 sion there given, I mentioned specially the curious construction of certain mounds 

 of limestone, which I called reef-knolls, and gave my reasons for supposing that they 

 bad been gradually built up on a slowly sinking sea bottom by the gradual 

 accretion of animal remains, somewhat in a similar manner to coral reefs. I also 

 showed that from the enormously disproportionate thickness of rocks in the area 

 of the ddwtithrow side and from other considerations there was every reason to 

 su'p'p'os'e^that the Craven Faults were actJually taking place during the formation of 

 thosefr'c'ckg. , . ,.,.,.. » i . 



My friend Mr. J. E. Ma'rr,F,R.S.; has in a inbst courteous way, whilst taking 



» Rr.pdri Brit. Assoc, is'so. 



