Gregory oil Diatomaceous Earth. 
93 
the Quaternary period, but he had only been able to make a 
partial examination of it at the time he wrote. 
This deposit must not be confounded with the Leaf-bed, 
also discovered in Mull by the Duke of Argyll ; for that bed, 
which also contains a large number of Diatomaceous remains, 
occurs at a place 20 miles from the deposit now under con- 
sideration, and is found between two beds of volcanic trap, 
showing that the Dicotyledonous trees — remains of which 
abound in it — must have lived before the eruption which gave 
rise to the upper trap bed, whatever may have been the period 
of that eruption. 
To return to the Infusorial deposit. The Duke of Argyll 
thinks it possible that the waters of Lock Baa, which now 
pass to the sea at a distance from the deposit, may, at one 
period, have flowed through the hollow where the deposit is 
found. Mr. Campbell Paterson, a gentleman residing on the 
spot, thinks that the sea at one time communicated with Loch 
Baa, and that the present barrier is the result of some geolo- 
gical change or convulsion. The gravel and sand, he says, 
exactly resemble those now forming in the neighbouring sea ; 
and although he has not observed any marine shells in the 
gravel, he thinks that the rocks at a higher level bear marks 
of the action of the sea. These are points on which I cannot 
speak without a personal knowledge of the locality, but the 
deposit appears to contain only fresh-water organisms. 
The Duke of Argyll kindly gave me a small portion of the 
earth first discovered, which happened to be very pure, and 
which he stated to contain Naviciilacece. On examining it, I 
was struck with the variety of forms, and resolved to study it 
more closely ; this I have only been able recently to do, and 
I think tlie results may prove not uninteresting to the Micro- 
scopical Society. 
The Mull earth is, in the purest specimens, when dry, 
almost white, and much resembles chalk, being light, friable, 
and adhering to the fingers. But more commonly it has a 
pale fawn colour, and it is frequently strongly tinged with iron. 
The lightest and whitest specimens contain hardly anything 
besides siliceous organic remains, for the most part entire, but 
with some fragments. Other portions, which are denser, 
contain also many fragments of quartz of various sizes, and 
vast numbers of comminuted fragments of lorica\ In the 
densest and worst, the quartz or sand and the fragments en- 
tirely predominate, and these can hardly be cleaned. The 
specimens of middling quality, as well as the inferior ones 
which I at present possess, contain a great many minute 
fragments of loricae, often exceeding half or three-fourths of 
