Mr. C. Babbage on the Parallel Roads of Glen Roy. 317 



the roads are formed was brought down by snow and ice slowly 

 descending; the hills until arrested on the margin of the frozen lake. 

 On the melting of the snow and ice, it was tranquilly deposited 

 without any further descent, and thus lay in a horizontal line. 



In conclusion the author adverted to the theory of the change of 

 isothermal surfaces within the earth, an account of which he had 

 published in the Society's 'Proceedings' for 1834, as affording the 

 necessary explanation of the causes which had produced the changes 

 of climate in the district of the Parallel Roads. 



2. " On the origin of smoothed, rounded, and hollowed surfaces 

 of Limestone and Granite." By D. Mackintosh, Esq., F.G.S. 



The author argued that smoothed surfaces are produced by the 

 action of water charged with sediment and stones, and that "hol- 

 lowed surfaces " are the result of coast-ice containing stones fol- 

 lowing the course of the advancing and retreating tide. 



3. " On a striking instance of apparent oblique lamination in 

 Granite." By D. Mackintosh, Esq., F.G.S. 



Lines of stratification and lamination were stated by the author 

 to be discernible in the granite rocks of Hountor, of the Kestor 

 group, and of the Blackstone rock ; and he added that they support 

 the idea of the aqueous origin of granite. 



4. " On the Encroachment of the Sea in the Bristol Channel." 

 By D. Mackintosh, Esq., F.G.S. 



Mr. Mackintosh has obtained information showing that at 

 Watchet, where the cliffs average 50 feet in height, the sea has 

 gained at least 200 yards during the last 150 years. He also 

 showed that at Weston-super-Mare the sea is converting the slopes 

 into cliffs, and he concluded with a notice of the caves and raised 

 beaches near that town. 



5. " On the two Plains of Hertfordshire and their Gravels." By 

 T. M'K. Hughes, Esq., M.A., F.G.S. 



The high ground near Hertford Heath, Brickenden, &c. forms 

 the higher of the two plains which Mr. Hughes described ; out of it 

 a great valley has been excavated, the bottom of which forms the 

 lower plain; and out of this, again, the valleys of the existing streams 

 have been scooped. 



The gravels of the upper plain are a marine deposit, and indicate 

 a marine denudation of great antiquity, followed by an emergence, 

 during which the old valleys were scooped out of that plain. The 

 gravels of these valley-plains were formed during a subsequent sub- 

 mergence ; they contain bands of clay and loam passing into 

 Boulder-clay, and are probably marine. This submergence con- 

 tinued until the Boulder- clay was deposited on the top of the higher- 

 plain gravels; and then succeeded a period of emergence, during 

 which the present valleys were scooped out of the lower plain. 



