174 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 17, 



retreat of the glaciers to higher levels during the return to a milder 

 climate, as, for instance, at Llyn Idwal* in Caernarvonshire ; while, in 

 the adjoining small lateral valley opposite the Penrhyn Quarry, I 

 have seen shells and granite brought up from the boulder-clay at a 

 depth of 30 feet; no traces, or very slight traces of it being found in 

 the main valley down which the glacier had descended. In Nantllo 

 Valley also, at the base of the steep escarpment of Llwydd-mawr, 

 there are two or three small mounds of the angular detritus of the 

 adjacent precipice which indicate a very small glacier, although it 

 hardly deserves that name, which may have reached their present 

 position on ice. I never could comprehend it while I thought only 

 of transport by aqueous action. Traces of the action of glaciers 

 during the period of emergence we can comprehend, while we doubt 

 the possibility of pointing out those which precede the period of 

 submergence. 



I question, however, if it be possible to point out the track of a 

 glacier belonging to the period which preceded the submergence. 

 The phenomena which have been mistaken for them are to be referred 

 to the action of heavy masses of floe-ice advancing into the interior 

 over sinking land. The period of submergence was one of foreign 

 erratics borne inwards ; the period of emergence, one of local er- 

 ratics borne outwards. 



The effects of terrestrial and marine ice of an arctic climate, whe- 

 ther we look to heated rocks, to scratched detritus, or to pot-holes, 

 are so closely similar, as I gather from the accounts of arctic voyagers, 

 that it is only by their relation to deposits containing shells, and by 

 taking general views which shall embrace the whole of the erratic 

 phenomena, including the presence of marine shells, and the condi- 

 tions under which they occur, that we can decide between the effects 

 of the action of terrestrial and marine ice. 



9. Note on the Bones of the Hind- foot of the Iguanodon, 

 discovered and exhibited by S. H. Beckles, F.G.S. By Pro- 

 fessor Owen, F.R.S., F.G.S. 



The specimens^ exhibited by Mr. S. H. Beckles, F.G.S., and dis- 

 covered by him in the Wealden-bed, south-coast, Isle of "Wight, con- 

 sist of bones of apparently the same individual of a more than half- 

 grown Iguanodon, including the metatarsals and phalanges of the left 

 hind-foot. This foot includes three toes. 



To the inner (tibial) side of the innermost of the fully-developed 

 metatarsals, there is attached a styliform rudiment of a more internal 

 metatarsal : the distal end of this rudiment is broken off, but it 

 gradually tapers to that end, and seems, like the splint-bones in the 



* See also Prof. Ramsay's remarks on the glacial phenomena of N. Wales, 

 Quart. Journ. Geol. Soc. vol. viii. p. 375. — Ed. 



f These specimens have been figured, and form the subjects of Plates 1, 2, 

 and 3 of Prof. Owen's " Monograph of Wealden Reptiles," published in the volume 

 of the Paljeontographical Society for 1856. 



