Repoiis and Proceedings — Geological Society of London. 95 



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 deposition 

 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 i-q- 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 sand- 

 stones 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, 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 tine- 

 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 45 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. 



