Mr Thomson on the Parallel Roads of Lochaber. 61 



Skye, in particular, among the Cuchullin Hills, which have 

 been lately explored by the last-mentioned author, Professor 

 Forbes, there are to be seen more striking and indisputable 

 traces of glaciers than in any other locality which has, as yet, 

 been examined. This is in a great degree to be attributed 

 to the durable nature of the hypersthene rocks of which those 

 hills are composed ; a property which has caused their sur- 

 faces to retain not only the general forms, but also the most 

 minute markings produced by the glaciers ; and which, at 

 the same time, has prevented these from being concealed 

 under a coating of decayed materials. The face of the country 

 seems, in fact, to have retained, almost absolutely unaltered, 

 all the appearances which it presented on the retiring of the 

 ice. 



In the Lochaber district, among other indications of the 

 action of glaciers, Agassiz has pointed out one which is in- 

 teresting in itself, and more so when taken in connection 

 with the foregoing. At the mouth of Loch Treig, the rock 

 consists of gneiss, intersected by veins of quartz. The quartz 

 everywhere projects two or three inches above the gneiss, its 

 upper surface being polished and striated, 'exactly as is the 

 case with quartz veins exposed to the action of glaciers at the 

 present day. It is clear that the gneiss and the quartz had 

 originally been planed down to one even surface ; and that 

 the gneiss, not being perfectly durable, has since decayed 

 away, and thus left the quartz veins standing in relief. 



It would be out of place for me here to enter at greater 

 length into the question as to the former prevalence of gla- 

 ciers, or of a glacial climate. For farther details, I must 

 refer to the authors who have discussed the subject, particu- 

 larly to those I have already mentioned. 



On Carbonic Acid as a solvent in the process of Vegetation. 

 By John Davy, M.D., F.R.S., Lond. & Ed., Inspector-Ge- 

 neral of Army Hospitals. Communicated by the Author. 



The importance of carbonic acid in the process of vegeta- 

 tion, as the principal source of the carbon of plants, is now 



