CONTAINING DIATOMACEOUS KXUVLE. '63 



it to have been formed in an estviarj, like the Thames miul, 

 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 snnd 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 geolog^ists 

 have arrived in many instances, from other phenomena, such 

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

 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 qurestiones vexatcB 

 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 possiblv carried by the 

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

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

 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, 

 when the quartz, mica, and other matters which separate them 

 are removed, a mixture of a very remarkable kind, in which 

 a large number of forms are tolerably abundant, and a still 

 larger number are pretty frequent, while none are so pre- 



VOL. III. D 



