300 ME. GEOEGE M. THOMSON ON A FEESHWATEE 



In trying to arrive at some conclusions as to the lengtli of time during wliicli 

 Anaspides has heen isolated, the following facts are of interest. I am indebted for this 

 information on the geology of the district to my friend and former pupil, Mr. A. Mont 

 gomery, M.A., Government Geologist of Tasmania. The top of Mt. Wellington, as well 

 as the central plateau of Tasmania, consists of a mass of greenstone (diabase) of about 

 Permo-Carboniferous age. According to Mr. R. M. Johnston, the greenstone existed 

 prior to the deposition of the shell-beds and sandstones which flank its base, while Messrs. 

 Stephens and Montgomery consider it to be younger. Mr. Johnston admits, how^ever, 

 that there was a " newer greenstone " eruption which broke through these marine 

 deposits, so that it is agreed that the Permo-Carboniferous beds were penetrated by 

 greenstones subsequent to their formation, and it is probable that the eruption of these 

 greenstones and the elevation of the marine beds were contemporaneous. The green- 

 stones break through and cover the Upper Coal-measures, W'hich have been referred to 

 the Mesozoic period on account of the plant-fossils contained in them, though no strati- 

 graphical break has yet been discovered between them and the Permo-Carboniferous 

 marine beds at a horizon very little below them. If Mesozoic at all, they must probably 

 be referred to the beginning of that period, and the greenstone eruptions were probably 

 little later. They were certainly older than the oldest Tertiary beds, which may be 

 Eocene. There are no Secondary marine beds, and the Tertiaries are quite littoral, so it 

 appears certain that Tasmania has never been under water to any great extent since some 

 time in the Mesozoic, and possibly since the close of the Palaeozoic period. It seems 

 probable that the basaltic eruption, of which the diabase greenstones are the remains, 

 took place early in the Mesozoic period, and that it was of immense extent, the whole of 

 the eastern half of the island being covered with lavas. Sub-aerial erosion has since 

 carved out these, leaving the central plateau and the top of Ben Lomond as remains 

 of the great covering sheet, and exposing the dykes and bosses through which tlie 

 molten matter was ejected, in the numerous small greenstone hills in the loAver country. 

 Mt. Wellington was probably one of the large centres of eruption, as a deep bore at the 

 Cascades near Hobart shows the greenstone to underlie the Permo-Carboniferous beds. 

 The fact that Tasmania has been dry land since Mesozoic times Avill probably not be 

 disputed by any one conversant wdth the geology of the country. 



It seems to me probable that Auasjndes has been isolated from some marine form 

 since a period shortly after the eruption of the greenstones referred to. The occurrence 

 of freshwater Crustacea belonging to marine types may be variously accounted for. It is 

 alw ays possible that the animals themselves or their eggs may have been carried by aquatic 

 birds ; but I know of no recorded case of such a mode of distribution, nor do I know of 

 any marine form which would survive in fresh water. I have often plunged various 

 littoral species belonging to different families (Palinurus, Atylus, Dexamme, Sphceroma, 

 &c.) into fresh water and found that they died in a very short period of time, usually in 

 the course of a few minutes. On the other hand, the adaptability of various marine 

 Crustacea to a freshwater habitat by gradual stages is a well-ascertained fact. The 

 following examples are of interest. In New Zealand I originally described Idotea laeiis- 

 tris from brackish water at the mouth of a lagoon which communicates with the sea at 



