Corresfondence — Rev. A. Irving. 45 



Dr. Hicks, added to that alreadj'- obtained, led irresistibly to one of 

 two conclusions ; — either that, when the Cambrian was formed, an 

 area of very ancient metamorphic rock was exposed near Ty Croes 

 and in the Caernarvonshire district, or that the rhyolitic volcanoes 

 were so much older than the Cambrian time that their granitic cores 

 were already laid bare by denudation. Hence, in either case, the 

 existence of Archcean rock in North Wales was proved. To one or 

 other of these conclusions he could see no possible alternative, and 

 he considered the former to be (even if some of the granitoid rock 

 w^ere granite) far the most probable. 



3. " On some Post-glacial Ravines in the Chalk-Wolds of Lincoln- 

 shire." By A. J. Jukes-Browne, Esq., F.G.S. 



In a former paper the author stated that of the valleys intersecting 

 the Chalk Wolds some were older and some were newer than the 

 formation of the Boulder-clays (Hessle and Purple Clays). He now 

 described some cases where the modern watercourse, after flowing 

 for some distance along the line of an ancient (pre-Boulder-clay) 

 valley, suddenly deserts that valley and passes through a ravine 

 excavated entirely out of the Chalk. 



These ravines are very different from the other parts of the 

 valley traversed by the same stream, being deep and narrow cuts 

 or trenches with steep wooded sides, and exhibiting more the scenery 

 of Derbyshii-e vales than of ordinary Chalk valleys. 



In accounting for the origin of these ravines, the author pointed 

 out that the whole district in which they occur must once have been 

 completely covered by the Boulder-clays; and he supposes that at 

 certain points where the ancient valleys were blocked with high 

 mounds of Drift, the streams found it easier to cut new channels 

 through the flanking ridge of Chalk than through the obstacles in 

 front of them. 



coiaiaiEsi^on^zDiEisros. 



THE PERMIAN-TRIAS QUESTION. 



Sir, — As the Permian-Trias question was brought forward rather 

 prominently (thanks to your courtesy) in the pages of this Journal 

 during last year, perhaps you will further allow me to make a re- 

 mark or two with reference to the paper bearing upon the subject 

 which I had the honour of reading this year before Section C. of the 

 British Association. The evidence which I was able to bring forward 

 (from recent work in Germany) as to the existence of local dis- 

 cordance and unconformity on a large scale between the Dyas and 

 Trias was admitted even by Prof. Hull to have fully established that 

 position. This, however, which was the main point, was not 

 noticed in any report of the discussion which I have seen in the 

 newspapers. It follows of course that wherever in my papers of 

 last year (following Murchison) I have spoken of a conformable 

 sequence between the Dyas and Trias of Central Europe, all that 

 must be considered now as imsaid. 



The retention of the name "Permian" after it has admittedly 



