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me, the material of which the deposit exhibited consists, and 
which encloses the well-preserved remains of Mollusca, is of 
more special interest, as this is the third time that such a 
formation has come under my observation, and I have studied 
one of these deposits with some care, as it proved to be, for 
the most part, made up of the Siliceous skeletons of Diatoma- 
cee, to the consideration of which, both recent and _ fossil, I 
have devoted many years. It is well known to Microscopists 
that the late Prof. W. Gregory had, in the year 1855, de- 
seribed what he had called a “ Post-Tertiary Lacustrine Sand,” 
containing Diatomaceous exuvia, from Glenshira, near 
Inverary, in Scotland. This sand contained the remains of 
both marine and fresh-water species of Diatomaces, which 
mixture has resulted from the fact that the spot in which it 
was formed, although a fresh water lake, was at times subject 
to the incursion of the water of the adjoining Loch Fine; 
perhaps during the high tides of spring and autumn. Prof. 
Gregory considered that the land had been raised since the 
formation of the deposit he examined, and such may have 
been the case. However this may be, we have in our own 
country a very similar deposit, which has been forming doubt- 
less for many years. This is the mud brought up from the 
bottom of Mystic Pond, near Boston, Massachusetts. This 
has been examined by Messrs. Stodder and Greenleaf, and 
found to contain the remains of both marine and fresh-water 
Diatomacee. The forms they found are enumerated in the 
Proceedings of the Boston Society of Natural History. In 
this ease the opening of the pond has a bar across it which 
prevents the ingress of salt water under ordinary cireumstan- 
ces. During high tides, however, the water of the Bay creeps 
up the Mystic River and flowing beneath the fresh water ot 
the stream itself, runs over into. the bed of the pond, so thar 
it is said to carry with it marine forms of life which remain, 
become acclimated, so to speak, and flourish at the bottom 
of the excavation. So it is well known that the salt wate 
creeps up beneath the fresh of the Hudson River, and at West 
Point certainly a mixture of forms are to be seen. Doubtless 
it was in this way that Baccillaria paradoxa, a brackish Diatom, 
