CONTAINING DIATOMACEOUS EXUVLE. 5 



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

 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 to suppose 

 it to have been formed in an estuary, like the Thames mud, 

 when we see a similar deposit in course of formation at the 

 present hour in the fresh-water lake, not much more than a 

 mile from the spot. 



But if this be admitted, then it must also follow that, since 

 the relative levels of sea and lake were the same then as now, 

 and since the sand occurs at a considerably higher level than 

 that of the present lake, — it must, I think, follow, that the sea 

 has fallen, or the land has risen, since the period when the 

 sand was deposited. This is a conclusion at which geologists 

 have arrived in many instances, from other phenomena, such 

 as raised beaches, as, for example, in the Clyde, with which 

 Loch Fine communicates. It is interesting to find the study 

 of the Diatomaceous forms, occurring so scantily in this de- 

 posit, assisting to throw light on one of the qucestiones vexatcc 

 of geology. ^ 



I have said that the Diatomes are but scantily diffused in 

 the Glenshira sand ; and this is true, since they do not much 

 exceed 1 per cent, of the mass. But when we examine the 

 purified or cleaned residue, in which they are, as it were, 

 concentrated, we are struck at once with the very large 

 number of species present. 



In this respect the Glenshira sand far surpasses every de- 

 posit hitherto described, even that of Mull, in which I have 

 found 150 species, and the Thames mud, in which Mr. Roper 

 detected 104 species. 



In the Mull deposit all the species, with a very few excep- 

 tions, and these so rare as to be evidently accidental, derived 

 from the proximity of the sea, and possibly carried by the 

 winds, belong to fresh water. But in the Thames mud and 

 in the Glenshira sand, as already stated, both classes of forms 

 occur abundantly. It is this which accounts for the large 

 number of species. Up to the present time I have recog- 

 nised in the latter not less than 240 species, and I am quite 

 satisfied that a good many remain to be identified. Judging 

 from what has been done already, I cannot doubt that the 

 number of species will^ before long, amount to at least 250. 



In consequence of the circumstances under which it has 

 been formed, this deposit does not contain any one or more 

 greatly predominant form, as is generally observed to be the 

 case in deposits formed where the Diatomes grew and died. 

 As they have all been transported by water, they constitute, 



