THE FRESH- WATER LOCHS OF SCOTLAND. 191 



north-east and south-south-west fault skirting the eastern shore, 

 whereby the Torridon Sandstone has been thrown down against the 

 Archaean floor on the east side. A glance at the map will show how 

 the contour-lines run parallel to that fault and close to the shore, 

 and that the deepest sounding, 153 feet, is not far from the line of this 

 dislocation. 



Loch Urigill and Loch Maol a' Choire are shallow rock-basins in the 

 Cambrian limestone, the erosion of which may be partly due to 

 solution. 



Loch Borralan lies along the line of a consequent valley, near the 

 headwaters of the river Kirkaig, and probably not far from the 

 original axis of uplift in early Palaeozoic times. It is a shallow rock- 

 basin, floored by igneous rocks which belong to the igneous mass of 

 Cnoc na Sroine, with some drift along its margin. 



From the evidence now adduced, it is obvious that the plateau of 

 Archaean gneiss with its intrusive dykes is dotted over with lakes of 

 various sizes, which, with the exception of the lower part of Loch 

 Assynt, are of moderate depth. Indeed, most of them are shallow 

 basins, which reflect the varying types of gneiss and intrusive dykes 

 and their subsequent deformation. It is, no doubt, true that the 

 numerous shear-lines and faults of pre-Torridonian age that traverse 

 the Archaean plateau have determined to some extent the features of 

 these lakes; but we are, nevertheless, of opinion that the evidence 

 taken as a whole is in favour of the theory that they have been mainly 

 produced by the erosive action of ice. 



NOTES ON THE BIOLOGY OF THE LOCHS IN THE ASSYNT DISTRICT. 

 By JAMES MURRAY. 



Only an incomplete sketch can be given of the biology of this 

 district, as collections of plankton were made in only twelve out of the 

 twenty lochs surveyed. Loch Assynt is the largest loch in the district, 

 but four others, viz., Lochs Lurgain, Skinaskink, Bad a' Ghaill, and 

 Owskeich, are moderately large and of considerable mean depth, so 

 that they fall to be classed rather with the large lochs than with 

 the small ones. The remainder are small or of low mean depth. 



The fauna of the plankton was very uniform throughout these lochs, 

 differing chiefly in the presence in some of them of one or other of the 

 northern species of Diaptomus, D. Wierzejskii, I), laciniatus, in the 

 Daphnia being D. lacustris in some and D. galeata in others, and in 

 the greater abundance of Rotifera and Rhizopods in the shallower 

 lochs. Though the three common Scottish species of Diaptomus were 



