Quantitative Study of the Structure of Rocks, 575 
made of similar materials may be used to determine the amount of 
vertical contraction of rocks since deposition, the average in cases 
studied in Tertiary and Secondary rocks being from 100 to 57. In 
studying the drifting of sand along the bottom by currents (on 
which the author experimented in a small stream many years ago), 
the results are found to vary, according to whether the water is 
depositing sand as well as drifting it, and according to whether ripples 
are or are not being formed on the bottom. The velocity of a current 
can be determined approximately in feet per second for different kinds 
ot sand. The connexion between the structure of 'ripple-drift' 
and time is discussed ; and an equation is given, from which the 
rate of deposit in inches per minute can be deduced. The con- 
nexion between the structure of a deposit and depth of water is 
found to be difficult to study quantitatively. From the occurrence 
of ' drift-bedding ' the depth of water may probably be determined to 
within a few feet, and on this being applied to particular rocks some 
interesting results come out, including the separation of sandstones 
into several different groups. The deposition of fine deposits, like 
clay, is a most complex subject, varying according to the amount 
of mud present in the water, and according to whether the grains 
subside separately or cohere together. When no pressure is applied, 
even when no further contraction takes place on standing for a 
year, the amount of water included in the deposited clay may be 
80 per cent., and, when dry, the minute empty spaces may still 
amount to 32 per cent. This leads to the conclusion that many of 
the older rocks must now be only 20 per cent, of their original 
thickness. In many cases there is produced by a gentle current a 
minute laminar structure from which probably the rate of depo- 
sition may be learned approximately, a common rate in the older 
rocks being from 9 to 18 inches per hour. But complex and difficult 
experiments are very desirable on this question. The rocks classed 
as clays differ very much in structure, and must have been formed 
under different conditions. 
Applying these conclusions to various rocks, the author shows 
that in the green slates of Langdale there is good evidence that 
the volcanic eruptions sometimes occurred within a few weeks of 
one another, and at other times at more distant intervals. Now 
and then there were bottom-currents, probably due to volcanic 
disturbances, gradually rising to a rate of about 1 foot per second 
and gradually subsiding, the entire period being a few minutes, 
and deposition taking place in different cases at from -^ to 2 inches 
per minute. There is also good evidence that, when deposited, 
part of the rock was analogous to fine, loose sand, and part to 
semi-liquid mud. In the Coal-Measure sandstones deposition at 
the rate of 1 inch per minute was common, with intervals of little 
or no deposit. 
The volume of invisible cavities in rocks varies from 49 per cent, 
in some recent rocks, to nearly in the ancient slates. The 
packing of grains is discussed mathematically and experimentally, 
