576 Geological Society: — 
the latter with round and flattened shot; and experiments "with 
sand of various qualities, rapidly deposited and also when well 
shaken, show a good agreement with calculation. The methods of 
determining the volume of minute cavities in rocks are given, 
followed by a number of examples from recent and older deposits. 
It is found that in some limestones the cavities have "been reduced 
by pressure to close on the mathematical minimum, whereas in 
others, even of Silurian age, the cavities were filled with carbonate 
of lime, introduced from without, not long after deposition. Some 
oolites have had their cavities filled in a similar manner ; in others 
most of the material of the original grain has been removed, and 
the present solidity is due to the filling-up of the original cavities 
mainly by internal segregation. Among fine-grained rocks the 
Chalk probably was originally a sort of semi-liquid with fully 
70 per cent, of its volume water, and in its present state is about 
4o per cent, of its original thickness ; the thickness of some clays 
must have diminished still more ; while the amount of minute 
cavities in rocks with slaty cleavage is so small, that sometimes they 
are nearly solid. 
By the measurement of green spots in slates it can be deduced 
that the rock before cleavage was somewhat more consolidated 
than rocks of the Coal-Measures now are, and was then greatly 
compressed and the minute cavities almost completely squeezed up. 
The development of * slip-surfaces ' in cleaved rocks is very great, 
and furnishes an additional proof that the cleavage is of mechanical 
origin. ' Pressure-solution' is also dealt with. 
In conclusion, the author discusses the volume of minute cavities 
in clay-rocks and their analogues of various ages, and shows that 
there is a distinct relation between it and the probable pressure to 
which the rocks have been exposed. Tables are given of the 
pressures so calculated for rocks of various geological ages, the 
volume of empty spaces decreasing in older rocks from the 32 per 
cent, existing in recent clays. In the ^loffat rocks, with very little 
or no slaty cleavage, the pressure is calculated at about 7 tons to 
the square inch, while the Welsh slates, with very perfect cleavage, 
indicate a pressure of about 120 tons to the square inch. 
January 22nd.— Sir Archibald Geikie, K.C.B., D.C.L., Sc.D., 
Sec.E.S., President, in the Chair. 
The following communications were read : — 
1. ' The Origin of the Pillow-Lava near Port Isaac in Cornwall.' 
By Clement Reid, F.E.S., F.L.S., F.G.S., and Henry Dewev, 
F.G.S. 
The Upper Devonian strata around Port Isaac consist of marine 
•slates, in which occurs a sheet of pillow-lava over 200 feet in 
