THE FRESH-WATER LOCHS OF SCOTLAND. 147 



south-west end, and has laid down a large delta, which is covered by 

 vegetation, thus contrasting with the other lake-shores, which are but 

 scantily clothed with vegetation. The Black burn, entering near the 

 middle of the southern shore, has not formed a delta, as it runs down a 

 rocky steep slope. The maximum depth is 256 feet, while the mean 

 depth is estimated at over 116 feet, and the volume of water at 2771 

 millions of cubic feet. The ratio of maximum depth to length is 46, and 

 of mean depth to length 101 ; these figures show the relatively great 

 depth of the basin, for in Loch Morar, the deepest of all British lakes 

 (Hi 1 7 feet), the ratio of maximum depth to length is 61, and of mean 

 depth to length 217, while the Lake of Geneva is 230 times longer than 

 deep. 



The basin is simple in conformation, the shores sloping on all sides to 

 the deepest part of the loch, which is centrally placed. At the south-west 

 end the oO-feet and 100-feet contours are sinuous, due to the deposition of 

 material brought down by the burns. The mud from the deepest part is 

 black and peaty. The approximate areas between the contour-lines drawn 

 in at equal intervals, and the percentages to the total 'area, are as 

 follows : ; 



Feet. Acres. Per cent. 



Oto 50 170 ... 31-1 ! 



50 100 96 ... 17'5' 



100 150 69 ... 12-5 



150 200 91 ... 16-5 



200 250 113 ... 20-7 



Over 250 ... 10 ... 1-7 



549 100-0 



These figures indicate the flat-bottomed character of the basin, the zone 

 between 200 and 250 feet being larger than any of the three preceding 

 shallower zones. 



The loch was surveyed on July 8, 1905, but the elevation could not be 

 determined from bench-mark ; from spot-levels it was estimated to be 

 about 1308 feet above the sea, and on the Ordnance Survey map the 

 elevation of 1309*8 feet is given, though the date when levelled is not 

 indicated. The surface of the water was 1-2 feet below the large stone at 

 the south-east corner of the pier at His Majesty's boathouse. The water 

 was then very low about 2 feet below the ordinary level and might rise 

 4 or 5 feet above ordinary level, the range being about 7 feet. 



Temperature Observations. At noon on the date of the survey the 

 temperature of the surface water was 58'2, and at 7 p.m. serial tempera- 

 tures gave the following results : 



Surface 56-l Fahr. 



50 feet 47-2 



100 44-3 



225 ,. 43-0 



