446 TRANSACTIONS OP SECTION E. 



over a tufa barrier which is being deposited by the waterfall. The barrier is 

 advancing down-stream, and the old gorge of the Kerka River, or as it is there 

 called, the Kerkic River, is accordingly being filled with alluvium. When the 

 falls have advanced another 500 yards, the river will leap from a hanging valley 

 on to the floor of the Knin Basin. 



The famous falls at Jajce, the ancient capital of Bosnia, are due to the leap 

 of the Pliva River, from a hanging valley over a tufa sheet, 80 feet thick, into 

 the Urbas River. Some Neolithic remains show that about 60 feet of the tufa 

 has been deposited at Jajce during and since Neolithic times. The Pliva River 

 has cut a notch through the old sheet of tufa which it had previously deposited, 

 owing to an increase in the gradient of the river caused by other tufa bars 

 erected further up its valley. 



These three waterfalls show that the ordinary waterfall action may be re- 

 versed ; waterfalls may advance instead of retreat, may fill up valleys instead 

 of excavating them, may form alluvial plains instead of destroying them, and 

 may make lakes basins instead of draining them. Waterfalls may also form 

 hanging valleys. 



2. Tilled Movements in the Deep Water of the Skagerrak and their Influence 

 upon the Herring Fishery. By Professor 0. Pettersson. 



The most prominent feature in the hydrographic state of the eastern side of 

 the North Sea, and its tributaries, the Baltic, the Kattegat, the Skagerrak, and 

 the Scandinavian fiords, is the stratification of the waters in layers of different 

 origin discernible by difference in their temperature and salinity. The boun- 

 dary surface between two adjacent water-layers can, as a rule, be located with 

 considerable accuracy, and by means of a registering apparatus changes in the 

 level of the deep water (as distinguished from the surface water) can be ascer- 

 tained. A study of these changes conveys the idea of a train of big sub- 

 marine or boundary waves. This wave movement seems to have its origin in 

 the deep water, as the surface water enters or is expelled from the fiord, when- 

 ever there is a subsidence or upheaval of the deep water. There are small boun- 

 dary waves of two to three days' period, which are evidently seiches of the deep 

 water inside the fiord, since they do not correspond to analogous movements of 

 the deep water of the Skagerrak outside the fiord, and there are great boundary 

 waves of thirteen to fourteen days' period, which are not confined to the fiord, 

 but are in most cases felt on the other side of the Skagerrak. The great boun- 

 dary waves seem to correspond in their period to the changes in the position 

 relatively to the earth of the sun and the moon; (a) the regular train of such 

 waves begins with the autumnal equinox and ends in the following April, the 

 biggest waves thus occurring when the earth is in perihelion; (b) the phases 

 of the moon, but still more its declination and distance from the earth, seem 

 to have influence upon the occurrence of the big waves. 



It seems obvious that the motions of the deep water are of a tidal nature, 

 and it seems possible that they belong to the special classes of such phenomena 

 called tidal seiches, i.e., movements of the waters of a limited basin caused by 

 the passages of the oceanic tidal waves over its threshold. To investigate this 

 theory, tank experiments were conducted, from which it would appear that the 

 original tidal wave, whether it be forced or free, is modified by its impact 

 upon every submarine ridge it encounters, and gives birth to tidal phenomena 

 of various kinds which can, however, always be recognised by their periodicity. 



The occurrence of big boundary waves seems to exercise an influence upon the 

 arrival of the herring shoals on the coasts of Sweden in autumn and winter. It 

 is well known that the fish life of the ocean has its highest frequency where 

 the movement of the waters is most intense. Investigations show that since 

 1753 the herring fishing has been most prolific in years of maximum declina- 

 tion of the moon and least prolific in years of minimum declination. A high 

 declination causes a more energetic movement of the waters of the innermost 

 parts of the North Sea and its inlets. In summer the herring shoals are spread 

 over the whole northern plateau of the North Sea, and in winter are found 

 concentrated in the eastern and southern parts. The chief agent is the sun's 



