THE FRESH- WATER LOCHS OF SCOTLAND. 283 



The loch is a flat-bottomed shallow basin, 45 per cent, of the lake-floor 

 being covered by more than 5 feet of water. The temperature of the 

 surface water on the date of the survey was 54-8 Fahr., while a 

 reading at the bottom in 7 feet gave 55*0. 



The particulars regarding the lochs in the Conon basin are collected 

 together in the table on p. 282 for convenience of reference and com- 

 parison. From this table it will be seen that in the sixteen lochs under 

 consideration, which cover an area of over 11 J square miles, nearly 

 2200 soundings were taken, or an average of 188 soundings per square 

 mile of surface. The aggregate volume of water contained in the 

 lochs is estimated at nearly 30,000 millions of cubic feet, and the 

 area draining into them is over 366 square miles, or 31J times the area 

 of the lochs. 



NOTES ON THE GEOLOGY OF THE CONON BASIN. 

 By B. N. PEACH, LL.D., F.R.S., and J. HORNE, LL.D., F.R.S. 



The rock groups entering into the geological structure of the Conon 

 basin and the area including Strath Glass and Strath Rusdale, north 

 of Ben Wyvis, belong to the crystalline schists and the Old Red 

 Sandstone. A line drawn from a point in Strath Rusdale above Ardross 

 Castle, south-west by Eileneach in Strath Glass, Achterneed station, 

 the Falls of Rogie, and across the Conon to Glen Orrin above Muirtown 

 House, roughly marks the boundary between the metamorphic rocks to 

 the west and the Old Red Sandstone bordering the Cromarty firth. 

 It will thus be seen that the crystalline schists form not only the greater 

 part of the basin, but also the highest and wildest territory. 



From the researches of the Geological Survey, extending over the 

 greater portion of the area under description, it would appear that the 

 metamorphic rocks may be arranged in two divisions : (1) a group of 

 acid, basic, and ultrabasic rocks, resembling certain types of Lewisian 

 gneiss of pre-Torridonian age along the western seaboard of Sutherland 

 and Ross; (2) the Moine series, representing altered sediments and 

 including the main subdivisions, (a) granulitic quartz-schists or quartz 

 biotite granulites, (b) flaky muscovite biotite schists or gneiss frequently 

 garnetiferous, and passing into flaggy mica-schists (pelitic schists). 



Though the group of rocks of Lewisian type comprises certain acid 

 granulitic gneisses that closely resemble the quartzose members of the 

 Moine series, yet their dominant feature is the alternation of acid and 

 basic materials in the form of biotite and hornblende gneisses. With 



