THE FRESH- WATER LOCHS OF SCOTLAND. 427 



(5) Brown Sand. 



To four samples of sediment dredged in 30 feet near the west coast 

 of Urquhart bay we give the name of Brown Sand. One of the samples 

 has the following composition : 



Minerals (69 per cent.), angular, mean diameter O2 mm., mostly 

 made up of quartz, coloured reddish by a coating of iron oxide. 

 Decomposed mica, hornblende, and plagioclase are also represented. 

 The sand contains a few small fragments of rocks, 1 to 3 millimetres in 

 diameter. 



Fine washings (31 per cent.), composed of vegetable matter (4-4 

 per cent.) and fine mineral particles (26-6 per cent.). There is no clayey 

 matter. 



Chemical Composition. 



Total silica 77'62 



Ferric oxide 3'60 



Alumina ... ... ... ... ... ... ... 5*20 



Lime 5'88 



Magnesia 2*20 



Loss on ignition 4'40 



98-90 



The alumina, lime, and magnesia are most likely due to the mica 

 and hornblende, whilst the defect of 1-10 per cent, might represent the 

 alkalies. 



Conclusion. 



Loch Ness includes two deep basins separated by a barrier formed by 

 the delta of the Foyers river. The muds from the south-western or 

 Invermoriston basin contain a large amount of vegetable or peaty 

 matter, brought down the lake probably by the rivers Tarff and Oich, 

 with mineral particles coming from the disintegration of the rocks, 

 transported by the streams. Small concretions of peroxide of iron and 

 dioxide of manganese were dredged at one station. The muds often 

 gave the characteristic reaction of manganese. On the slopes the muds 

 are sandy, and of a red-brown colour, due to the presence of oxide of 

 iron. 



The muds from the north-eastern or Urquhart basin contain far 

 less vegetable matter than those from the south-western basin, which 

 may be due to the Foyers barrier retaining the vegetable matter in 

 the upper basin. In the north-eastern basin the vegetable matter 

 increases with the depth, which is contrary to what is observed in the 

 south-western basin. Off Urquhart bay the contour-lines approach 

 each other very closely, and the vegetable matter brought down the lake 

 by the river Enrick is carried towards the deeper part of the basin. 

 Great differences are observed in the muds from the slopes on the two 

 sides of the loch. On the north-western slope we find especially a red 



