574 Corresjjondence — Mr. A. Strahan. 



beds of Central Wales are quite as metamorpliic-looking as much 

 of the " gnarled series " of Anglesey. We are presented in that 

 island with as complicated a piece of geology as Great Britain can 

 show, and no little field-work must be patiently prosecuted before 

 the problems can be solved. 



According to my view, Dr. Callaway has misapprehended some of 

 the most important sections. In a short paper printed in this 

 Magazine last March, I pointed out that the Nebo sections described 

 by him as unconformable junctions of " Ordovician shales" on 

 granitoidite are really faulted junctions of shales against the base- 

 ment bed of the Cambrian. This at Nebo is a very compact fine- 

 grained grit, which Dr. Callaway has mistaken for granitoidite. At 

 Bryngwallen quarry, near Llanerchymedd, a preciselj^ similar grit 

 may be seen passing down into a quartz conglomerate not distin- 

 guishable from that of Twt Hill, and passing up into a fossiliferous 

 sandstone containing OrtJiides, the whole section included in some 

 30 or 40 feet. E. D. Egberts. 



Clare College, Cambridge, Hov. 7, 1881. 



THE "LOWER KEUPER SANDSTONE" OE "BASEMENT BEDS." 



Sir, — While thanking Mr. Wilson for his support of much that I 

 have said on these rocks, I must correct a slight misapprehension 

 Avith regard to ray views. I do not hold the "theory of a great 

 break" at the base of the Waterstones attributed to me, but merely 

 point to the recurrence of lines of erosion at this and other horizons 

 in the Trias to show that they are no evidence of want of con form - 

 ability; on p. 6 I use the words "though there is no unconform- 

 ability," etc. 



I do, however, believe that a great change of physical conditions 

 commenced at this period, and that, judging by its effect upon the 

 nature and distribution of the deposits, this was the most important 

 change that took place in the British region during the Triassic era. 

 For I consider the theory that the Bunter was upheaved into dry land 

 and denuded, before the Keuper was deposited, far from being proved. 



Mr. Wilson states that " At the close of the Bunter period eleva- 

 tion took place, in the Midlands certainly, if not generally thi'ough- 

 out the country, accompanied by extensive and long-continued 

 denudation." The evidence for this elevation and denudation con- 

 sists in the fact that the Keuper Basement Beds rest on Pebble Beds 

 near Nottingham, but on Lower Mottled Sandstone at four miles 

 distance, the inference being that at least 200 feet must have been 

 denuded away. 



But it must be borne in mind that the Bunter deposits thin away 

 to the south-east, as though deposited against a shelving shore, and 

 that Nottingham stands on the margin of the area over which these 

 shingles were originally spread. The inference therefore that the 

 Pebble Beds must have been denuded away in those places where 

 they are absent below the Keuper is unsafe, for they probably never 

 extended so far. It is true that the disappearance of the Pebble 



