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XXXI. — On New Forms of Marine Diatomacew, found in the Firth of Clyde and 

 in Loch Fine. By William Gregory, M.D., F.R.S.E., Professor of Che- 

 mistry. Illustrated by numerous Figures, drawn by R. K. Greville, LL.D., 

 F.R.S.E. 



(Read 19th January 1857.) 



In two papers read before this Society, I have very fully described the Diato- 

 macese of the Glenshira Sand, which is very remarkable both for the large number 

 of species found in it, which is certainly more than 320, and for the circumstances 

 in which it must have been deposited. There can be no doubt, from the nature of 

 the locality, which I have lately visited, that this bed was formed in the bottom of 

 the Dhu Loch, a shallow fresh- water lake, at that time extending about two miles 

 farther up the valley than it now does, and being at a higher level. In consequence 

 of a rise in the level of the land, or a fall in that of the sea (from which — that is, 

 from Loch Fine, the lower end of the lake is separated by a narrow and low bar- 

 rier, through which the waters of the lake pass to Loch Fine), the lake has long 

 ago been drained, till its upper end is nearly two miles from the point it must have 

 reached when the bed of sand was formed. The present level of the lake is con- 

 siderably lower than it was then ; the precise difference I had no means of ascer- 

 taining, but I believe it is about 30 feet. Now, the most interesting fact about this 

 lake is, that its actual level is that of half-tide, so that at low water the lake is 

 discharged into the sea, while at high water the tide flows upward into the lake. 

 Hence marine plants and animals are found in the Dhu Loch ; herring, for ex- 

 ample, are often caught in it, and were taken while I was in the neighbourhood. 

 Hence also the present deposit in the lake exhibits a mixture of fresh- water and 

 marine Diatomaceous forms. Now, the older sand, the subject of my paper, de- 

 posited at a considerably higher level, also contains both marine and fresh-water 

 Diatoms ; and while the individuals of the two classes are both abundant, the 

 marine species are at least twice, perhaps thrice, as numerous as those of fresh 

 water. 



The natural, and, I have no doubt, the true explanation of the occurrence of so 

 many marine forms in an inland deposit, formed in a fresh- water lake, is this : 

 that at the period when the sand was formed the relative levels of the Dhu Loch 

 and of Loch Fine were the same as now, when similar results ensue. 



But as the lake was then at a higher level than now, so also must the sea 

 have been at a level as much above its present one. This conclusion is in ac- 

 cordance with those derived from the observations made on raised beaches on the 



VOL. XXI. PART IV. 6 M 



