io64 PRINCIPLES OF STRATIGRAPHY 



Champlain in this group, but this basin can hardly be said to have 

 been a part of the sea in the sense that the marginal fjord lakes 

 are. It is an ancient erosion basin of truly continental origin, as is 

 the case with most of the Great Lakes. The Champlain valley was 

 temporarily invaded by the sea, as was Lake Ontario, both being 

 primarily continental basins due to erosion and damming. For this 

 group a separate division — that of relict seas due to invasion — 

 might be erected. In this category may belong Lake Venern and 

 Lake Vettern of Sweden, both of which are old erosion basins 

 and may have been lakes before the marine invasion. This group 

 of relict seas is more nearly related to the true lakes which contain 

 a relict fauna. 



Among other lakes containing a pronounced relict fauna, Lake 

 Baikal of central Asia and Lake Tanganyika of central Africa 

 are perhaps the best known. The relict nature of the former basin 

 has been much questioned (Credner-8, ii:^5), but the relict nature 

 of a part of the fauna is commonly conceded (Hoernes-22). Be- 

 sides the seal Phoca haikalcnsis, B. Dyb., and a number of fish, 

 among them Saliiio migratorius Pallas, planarians, and sponges are 

 represented by types more closely related to marine than to fresh 

 water forms. One of the sponges especially (Lubotnirskia haika- 

 lcnsis, W. Dyb.) is a truly marine type, occurring in Behring Sea. 

 The molluscan fauna of Lake Baikal is especially peculiar. Of the 

 twenty-five described species of gastropods only three are found 

 elsewhere, but none outside of Sil)eria (Dybowski, W.-13). There 

 is a marked resemblance, however, between this fauna and that of 

 the Tertiary fresh water lakes of eastern Europe, which most 

 probably were the source whence the Baikal fauna migrated. 



The relict fauna of Lake Tanganyika is a most interesting and 

 remarkable one (Moore-32). It includes the fresh water medusa 

 Liuinocnida tanganyiccc, the first and almost the only jelly-fish found 

 outside of the sea; and a number of molluscs which can be traced 

 back to ancestors probably in the Jurassic seas of that region. 

 Some of these would unhesitatingly be classed as marine types if 

 found in a fossil condition, which is particularly true of the Fulgur- 

 like genus Holacantha, of which four species inhabit this lake; and 

 of the trochoid genus Limnotrochus, also represented by four spe- 

 cies (Bourguignat-4). This association, with Planorbis, Physa, 

 Vivipara, and other normal fresh water forms, would, if found 

 embedded in the strata, lead to some interesting speculations re- 

 garding the conditions of deposition. 



The relict fauna of the Scandinavian and Finnish lakes, of 

 which there are no less than thirty-one, comprises seven Crustacea, 



