THE SCOTTISH GEOGRAPHICAL MAGAZINE. A BATHYMETRICAL SURVEY OF THE FRESH-WATER By Sir JOHN MURRAY, K.C.B., D.Sc., F.R.S., and the late PART III. THE LOCHS OF THE TAY BASIN. 1. Lochs Ericht and Garry. INTRODUCTION. It is proposed to publish the results obtained in the lochs forming part of the drainage basin of the Tay as ready, and in this first instal shall deal with Lochs Ericht and Garry. The general conclusions will be reserved until the lochs of the whole basin have been surveyed. We have referred in Part I. to Mr. Grant-Wilson's work on some of the lochs of the Tay basin; he has kindly supplied us with copies of his maps showing the soundings taken by him. We propose as opportunity offers to take soundings supplementary to, and corroborative of, those taken by Mr. Wilson, and indeed we have already taken about 150 soundings in Loch Earn. The geology of the district around Lochs Ericht and Garry has not yet been systematically worked out, and therefore any notes by Messrs. Peach and Horne will be reserved until the completion of the survey of the lochs in the basin. MAPS AND ILLUSTRATIONS. This part is illustrated by three maps, and a temperature diagram. The maps are drawn to the same scale as has been adopted for the whole series, viz., three inches to the mile (1:21, 120), the majority of the VOL. XVII. N soundings being given in feet. The intervals between the contour-lines of depth are indicated by different shades of blue, and the intervals between the contour-lines of height of the neighbouring country by shades of brown. Plates I. and II. show Loch Ericht, which, in order to avoid an extremely long map, has been cut into two portions, the contour-lines of depth being drawn in at 50, 100, 200, 300, 400, and 500 feet. Plate III. is a longitudinal section of Loch Ericht, showing the distribution of temperature as observed on June 15th to 20th, 1900; this will be referred to in greater detail under temperature observations. Plate IV. shows Loch Garry, the contour-lines of depth being drawn in at 25, 50, 75, and 100 feet. Depths of THE LOCHS. Loch Ericht.-Loch Ericht is a large Highland loch situated partly in Perthshire and partly in Inverness-shire, at a high elevation among the Grampians. It is one of the wildest and most magnificent lochs of Scotland, presenting all along its shores scenes of lonely grandeur and sublimity, the mountains rising from the water's edge to great altitudes, their sides scarred by mountain torrents. The surface when measured by the Ordnance Survey officers in 1872 was found to be 1,153 4 feet above the level of the sea; it is thus one of the most elevated of the larger Scottish lochs. It is known to anglers as the home of large Salmo ferox, as well as of trout said to be equal in quality to those of Loch Leven. It trends in a north-east and south-west direction, and is broadest near the southern end, narrowing gradually towards the northern end. It is over 14 miles in length, and over one mile in maximum breadth; the mean breadth is about half a mile, being 3 per cent. of the length. Its waters cover an area of over 4600 acres (or nearly 7 square miles), and it drains an area seven times greater, or over 32,000 acres (nearly 50 square miles). The total number of soundings taken in Loch Ericht was 488, which show that it is a comparatively deep loch, the greatest depth observed being 512 feet. The mass of water contained in the loch is estimated at 38,027,000,000 cubic feet, and the mean depth at 189 feet, being 37 per cent. of the maximum depth. The length of the loch is 150 times the maximum depth, and 405 times the mean depth. The deepest part of the loch is in the southern broader portion, where, about 3 miles from the foot of the loch, there is a small central depression, about one-third of a mile in length, and covering about 58 acres, in which the depths exceed 500 feet, the maximum being 512 feet. There are two 400-feet depressions, the larger, about three miles in length, reaching to about 1 miles from the southern end, and enclosing the 500-feet depression. Separated by about a quarter of a mile from the northern end of the large 400-feet depression is the second smaller isolated depression, in which the maximum depth is 410 feet. There are two 300-feet depressions, the larger in the southern portion of the loch, the smaller in the northern portion. The southern depression is over 4 miles in length, and encloses the deepest water in the loch. The northern smaller depression is under one mile in length, with a maximum depth of 314 feet, and approaches to within 2 miles of the head of the loch. There are two 200-feet depressions; the larger runs from within a mile of the southern end to more than half way towards the northern end, being over seven miles in extreme length. It is separated from the northern 200-feet depression by an interval of 23 miles, in which the depth varies from 127 to 194 feet. The northern 200-feet depression is nearly 23 miles in length, approaching to within about 1 miles from the northern end, and enclosing the small northern 300-feet depression already mentioned. The 100-feet depression is a continuous area extending from within less than half a mile of the southern end to within less than a mile of the northern end, and is about 134 miles in total length. The 50-feet depression follows approximately the contour of the loch. Opposite Loch Ericht Lodge an isolated sounding of 44 feet was observed between the 50- and 100-feet lines, and about 1 miles farther down, opposite the entrance of the Allt Camus nan Cnàmh, another isolated sounding of 20 feet was taken, surrounded by deeper water. The area of the bottom of the loch lying between the shore and the 50-feet line is estimated at about 880 acres (or 19 per cent. of the total area of the loch), that between the 50-feet and 100-feet contours is estimated at 695 acres (or 15 per cent.); that between the 100- and 200-feet contours is estimated at about 1160 acres (or 25 per cent.); that between the 200- and 300-feet contours at about 875 acres (or 19 per cent.); that between 300 and 400 feet at 476 acres (or over 10 per cent.); that between 400 and 500 feet at about 474 acres (or over 10 per cent.); and that over 500 feet at 58 acres (or 14 per cent.). It will thus be seen that Loch Ericht is of very simple conformation. The deeper parts are divided into two basins, by the constriction in the outline of the loch in the vicinity of Loch Ericht Lodge, where, in one place, it is less than a quarter of a mile in width, but even here the depth in the centre exceeds 100 feet. Loch Garry.-Loch Garry 1 lies to the east of Loch Ericht, at a still higher elevation, and the scenery round about is very wild; the height of the surface of the loch above sea-level is not given on the Ordnance Survey map, but a height of 1326 feet is shown near the outlet, so that the level of the loch is probably about 1320 feet above the sea. In trend and in outline it somewhat resembles Loch Ericht, narrowing towards the northern end. It is over 2 miles in length, the maximum width being over a quarter of a mile; the mean breadth is slightly under a quarter of a mile (being 9 per cent. of the length). Its waters cover an area of about 390 acres (three-fifths of a square mile), and it drains an area thirty-seven times greater (or about 22 square miles). The total number of soundings taken in Loch Garry was 141, the maximum depth observed being 113 feet. The mass of water contained in the loch is estimated at about 846,000,000 cubic feet, and the mean depth at nearly 50 feet 1 This loch must not be confounded with the larger Loch Garry in Inverness-shire. (being 44 per cent. of the maximum depth). The length of the loch is 119 times the maximum depth, and 260 times the mean depth. Loch Garry forms a simple basin, except that the bottom sinks into two depressions exceeding 100 feet in depth, separated by depths of 82 to 93 feet. The larger but shallower depression is situated in the southern half of the loch, and is over a quarter of a mile in length, the maximum depth observed therein being 105 feet. The smaller but deeper depression is situated in the northern half of the loch, being only about one-sixth of a mile in length, and containing the maximum depth of the loch-113 feet. The 75-feet, 50-feet, and 25-feet depressions. form continuous areas, following approximately the outline of the loch. The 75-feet depression is nearly 1 miles in length, the 50-feet depression nearly 2 miles in length, and the 25-feet depression 21 miles in length. The area of the bottom between the shore and the 25-feet contourline is about 117 acres (or 30 per cent. of the total area of the loch); that between the 25- and 50-feet contour-lines is about 83 acres (or 21 per cent.); that between 50 and 75 feet is almost the same; that between 75 and 100 feet is about 87 acres (or 22 per cent.); and that over 100 feet is about 19 acres (or 5 per cent.). The details regarding the physical features of Lochs Ericht and Garry are collected together in the table on the opposite page. DEPOSITS. The deposits from Loch Ericht are interesting because of the evidence of layers of different colours. At a depth of 50 feet a red sandy mud was obtained; at. 112 feet the mud was white beneath and brown on top; at 124 feet it was all brown; at 153 feet all brown; at 182 feet sandy and white; at 184 feet white and brown; at 245 feet the deposit was a light-coloured mud, with a thin brown layer one inch in thickness on the top; at 270 feet it was white below, black-brown above; at 366 feet the mud was all dark-brown; at 385 feet there was a white clay or mud, with a dark layer on the top; at 456 feet the mud was all black; at 497 feet a section of black mud 5 inches in thickness was obtained; and at 510 feet the same black mud was found, without any trace of the lightercoloured mud. The sand from 50 feet consisted largely of mineral particles (probably 70 per cent. of the whole deposit) with a mean diameter of about 0.6 mm., one or two rock fragments attaining a diameter of 7 mm. The remainder of the deposit consisted of clayey and vegetable matter, with minute mineral particles less than 0.05 mm. in diameter, Diatoms, Sponge spicules, and Entomostracous skeletal remains. The light-brown mud from 150 feet contained about 30 per cent. of mineral particles, with a mean diameter of 0.5 mm., the largest being 5 mm. in diameter, with clayey and vegetable matter, and organic remains as previously mentioned. The dark-brown mud from 366 feet contained only about 10 per cent. of mineral particles (quartz, black and white mica, etc.) exceeding 0.05 mm. in diameter, the mean diameter being about 0.2 mm. Samples |