32 ON A POST-TERTIARY LACUSTRINE SAND, 



as far as concerns the Diatomes present, between this sand 

 and the Thames mud recently described by Mr. Roper in the 

 second volume of the " Journal." 



The first glance at the Glenshii'a sand under the micro- 

 scope leads to the observation, that, like the Thames mud, it 

 contains both marine and fresh-water forms. In this respect 

 it resembles the deposit or mud of all estuaries. From its 

 position, however, there is every reason to conclude that it 

 was formed in its present locality, when that part of the valley 

 was occupied by a fresh-water lake, which is now confined to 

 the lower part of the valley, but has evidently extended much 

 higher in former periods. The question of course naturally 

 occurs, whence came the abundant marine forms? But this 

 is easily explained, if we attend to what is going on in the 

 present small fresh-water lake. The level of this lake is pre- 

 cisely that of half tide, so that at high water the sea flows 

 into it, while at ebb tide the water of the lake runs into the 

 sea. 



This remarkable state of matters produces a mixture, in the 

 lake, of fresh-water and marine forms, both animal and vege- 

 table. The Duke of Argyll mentions, that nets, thrown for 

 salmon in the lake, have been drawn up full of herring ; that 

 other marine animals occur in it, and that marine algae are 

 also found, dwarfed by the influence of the fresh water. 

 Having been supplied with some of the deposit or mud now 

 forming in the lake, I examined it, and found it very closely 

 to resemble the sand from the higher level, save that the pro- 

 portion of organic matter was considerably greater. But, like 

 the older sand, it contains both marine and fresh-water 

 Diatomes, and these belong in many instances to the same 

 species. I have noticed some difference in the relative pro- 

 portions of species, and I shall take an opportunity of care- 

 fully studying the recent deposit or mud of the lake ; but in 

 the mean time I can state, that in all essential characters the 

 recent deposit agrees with the fossil one. 



From these facts it may be inferred that the lacustrine sand 

 of Glenshira, which I refer to the post-tertiary period, on 

 the authority of the Duke of Argyll and of Mr. Smith of 

 Jordanhill, both of wliom are familiar with the localities, was 

 formed in the lake when that lake occupied the part of the 

 valley where the sand occurs, and that the relative levels of 

 lake and sea were then the same as now. This seems to be 

 the simplest mode of accounting for the abundance both of 

 fresh-water and of marine forms. Had the sand been depo- 

 sited in sea water, it could not have been, as it is, extremely 

 rich in fresh-water species, and there is no reason t(> suppose 



