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Proceedings of the Royal Society 
on which, therefore, carbonic acid could produce no effect. They 
have this further advantage, that the marine and freshwater species 
each keep to their own distinct localities, and if any such fossils, 
therefore, could be found in the parallel roads, they might give 
important assistance in deciding between the marine and fresh- 
water theories. 
Accordingly, in the autumn of 1872, and again in 1873, an 
attempt was made to obtain fossil diatoms from these deposits. It 
was found that in Grlen Roy there were a good many points where 
the parallel roads were cut through and laid open. Four of these 
8 in. to 1 foot. 
2 to 3 feet. 
3 in. to 8 in. 
about 20 feet going 
down to the rock. 
sections were examined with some care, and one was especially 
fixed on lying high and dry on the hill side, where the internal 
structure of the lowest terrace was distinctly shown. The object 
was to obtain specimens of diatoms which may have been alive 
when the terraces were formed, and which were then included in 
the deposits; but to secure this, various precautions were necessary. 
The nature of the different beds composing these terraces will be 
* It may be a question whether this bed d really belongs to the time of 
the lowest shelf, or whether it is not some anterior formation — the sloping 
bottom of the lake, perhaps, at some earlier time. 
a, Humus — peaty, 
b, Stones with clay, 
c, Finely stratified sand and clay, 
d , Clay with boulders indistinctly 
stratified, with thin irregular 
courses of sand,* . 
