428 BATHYMETRICAL SURVEY OF 



sandy mud, coming without doubt from the washing out of the shore, 

 composed of Old Red Sandstone. On the south-eastern slope we have a 

 fine yellow clay, with fragments of rocks and large mineral particles 

 without vegetable matter. The deposition of the clay in this position 

 may be due partly to the strong prevailing westerly winds of Loch Ness 

 giving rise to waves and currents, which would carry the fine clayey 

 matter brought down by the Inverfarigaig river towards the south- 

 eastern shore. Three stones from a depth of 100 feet, opposite Inver- 

 farigaig pier, were covered with a dark ring of manganese dioxide, 

 marking out the line between the mud and water, as was pointed out 

 by Sir John Murray and Mr. Robert Irvine in their valuable paper : 

 "On Manganese Oxides and Manganese Nodules in Marine Deposits,"* 



MIRAGES ON LOCH NESS. 



A kind of mirage is one of the most familiar phenomena on Loch 

 Ness, especially in winter and spring. It is best seen in the morning. 

 Distant objects, such as the steamers plying on the lake, appear as 

 though raised above the surface and floating in the air. 



The most constant feature of the Loch Ness mirages is seen at 

 promontories some miles distant. The shore-line at the promontories, 

 though really nearly parallel with the horizon, is caused by the mirage 

 to appear to form an angle with the horizon. When this angle is great 

 (say 60 or more), the promontories appear like overhanging cliffs. 

 When the angle is very acute, they seem to be suspended over the 

 horizon. Objects which are known to be below the horizon are brought 

 into view. The receding steamer, after sailing out of sight, will 

 reappear miles further away, raised high above the loch and looking 

 very large. The promontory at Dores appears as a conspicuous island 

 in the middle of the loch. The fathers in the Benedictine Monastery at 

 Fort Augustus tell that on one occasion a snow-covered mountain 

 appeared over the end of the loch. These phenomena are best marked 

 at a distance of several miles from the observer. The steamer, sailing 

 away from the observer, seemed, at the distance of a mile or more, to 

 leave the surface of the loch and sail up into the air. Signs of the 

 mirage were sometimes to be distinguished at lesser distances. Stand- 

 ing on the deck of the Lake Survey yacht Rhoda, when the eyes would 

 be 7 or 8 feet above the water, there could often be seen on the rocks 

 of the nearest parts of the shore a conspicuous horizontal line, looking 

 just like a high-water mark. 



* Trans. Roy. Soc. Edin., vol. 37. p. 721 (1894). 



