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XXXIL—On New Forms of Marine Diatomacee, found in the Firth of Clyde and 
in Loch Fine. By Witttam Grecory, M.D., F.R.S.E., Professor of Che- 
mistry. Jllustrated by numerous Figures, drawn by R. K. Grevitwe, LL.D., 
F.R.S.E. 
(Read 19th January 1857.) 
In two papers read before this Society, I have very fully described the Diato- 
mace 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 ata 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 ofthe 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. 6M 
