Sir H. H. Howorth — Geological History of the Baltic. 355 



is formed of two ovals separated by a narrow stricture between the 

 towns of Umea in Sweden and Vasa in liussia, where the Archipelago 

 of Quarken is situated. The more northern of these ovals, with an 

 area of about 662 square miles, is known to the Swedes as Botten- 

 viken, while the southern one, with about 1,215 square miles, is 

 known to them as Bottenhavet. The former is now virtually a fresh- 

 water inlet. It contains but one living marine mollusc, and this 

 only in its extreme southern part, namely, Macoma soUdula^ TelUna 

 Balthica, which Nordquist reports from lat. 63-52 N. 



As we have seen in previous papers, the latest raised beaches all 

 round the coast of the Bothnian Gulf, including those at its head, prove 

 that before the last changes of level took place in its shores there were 

 four marine molluscs living there which have all now migrated further 

 south, owing to the sweetening of the waters, caused largely by the 

 inflow of the rivers having dominated that from the North Sea. 



The reduction of the salinity may be measured by the fact that 

 two of these molluscs, Litorina litorea and L. rudis, are both 

 very adaptable littoral shells, seldom found at a greater depth 

 than the low-water mark of spring tides, and often in large numbers 

 in hollows of the rocks above the highest tides. Gwyn Jeffreys 

 found the former living on the shore in a stream of perfectly fresh 

 Avater during the recess of the tide {Brit. Moll., ii, 106). The latter 

 is often found in places overflowed by freshwater streams during the 

 recess of the tide with its companions, the common mussel and the 

 limpet (ibid., iii, 267). This means that they can live wliere 

 the water is at one time fresh and at another salt, but not where, as 

 in the Bothnian Gulf, the percentage of salt is always very small. 



Both species occur in the lower raised beaches of the Baltic, and 

 Litorina rudis has been found in them at IS'eder Kalix at the very 

 head of the Bothnian Gulf. From them, as characteristic shells, 

 these beaches have been called Litorina beaches, and the Baltic, at 

 the time when they formed its margins, has been named the Litorina 

 sea. In their strict sense the Litorina sea and the Litorina period 

 came to an end when the uplifting of its bed and borders led to the 

 shrinkage of the water from the area marked out by the Litorina 

 beaches to its present contour. The change was limited to the 

 restriction of its area and the re-arrangement of the range of dis- 

 tribution of its living contents, otherwise the Litorina sea had 

 a continuous life with the present Baltic. This I have tried to show 

 was not the result of a gradual change of level but of a spasmodic one. 



The present conditions in regard to salinity and the present 

 distribution of the mollusca, in the latitude of Bornholm represent 

 very nearly what these elements were at the head of the Bothnian 

 Gulf in the Litorina time. 



As I pointed out in the second paper of this series, the Litorina 

 sea was the successor of a great freshwater lake whose limits 

 are marked out by the upper beaches of the present Baltic, and 

 which contain no debris of marine life but only freshwater remains. 

 From a notable shell the lake contained, it has been called the 

 Ancyliis lake or sea, which in turn gave its name to the Ancylus 

 period. All this is now universally accepted and has been amply 



