Post-glacial Ravines in the Chalk- Wolds of Lincolnshire. 73 



granitoid and gneissic rocks which occur in situ between that place 

 and Ty Croes, and that the matrix contained smaller fragments, 

 probably from the same rock, with schist bearing a general re- 

 semblance to members of the group of schists so largely developed 

 in Anglesey, and with grits, argillites, &c. Pebbles of granitoid 

 aspect in the Cambrian conglomerate near Dinas Dinorwig &c. bear 

 a very close resemblance to the Twt-Hill rock, and are associated 

 with abundant rolled fragments of rhyolite resembling those already 

 described from the Cambrian conglomerate and the underlying con- 

 glomeratic beds and rhyolites. Two pebbles of rather granitoid 

 aspect in the Cambrian conglomerate by the shore of the Henai 

 Straits, near Garth, prove to be spherulitic felsite, somewhat re- 

 sembling that already described by the author from Tan-y-maes. 

 He pointed out that the evidence of these specimens collected by 

 Dr. Hicks, added to that already 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 vol- 

 canoes 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 Archaean rock in North Wales was 

 proved. To one or other of these conclusions he could see no pos- 

 sible alternative, and he considered the former to be (even if some 

 of the granitoid rock were granite) far the most probable. 



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

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



In a former paper the author stated that of the valleys inter- 

 secting 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 Derbyshire vales than that 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. 



