﻿Vol. 64.] QUANTITATIYE METHODS TO THE STUDY OP KOCKS. 191 



dependent on so many variable conditions, that one might feel in- 

 clined to abandon the enquiry, were it not that so much of the 

 history of our rocks appears to be written in this language. 



The method employed in my experiments was to break up 

 thoroughly in water some fine-grained yellow clay from my garden, 

 due to the decomposition of a Coal-Measure shale, and, allowing the 

 coarse matter to subside, to pour the finer into beakers of various 

 sizes, with nearly perpendicular sides. After the contents of these 

 had been well stirred up, the extent of the subsidence was carefully 

 marked and measured at equal intervals, at first of a quarter of an 

 hour, and later of one and two days. 



The laws regulating the deposition and consolidation of clay in 

 still water are very complex, and differ completely according to the 

 relative amount of the solid material. When this is less than 

 about 1 per cent., the particles remain separate, and appear to 

 subside with a final velocity depending on their size and density. 

 The result is that, in the earlier stages, the coarser grains collect 

 at the bottom, and the supernatant water is not clear near the 

 surface, but is increasingly muddy downwards. On further standing, 

 finer and finer-grained mud reaches the bottom, and the water 

 becomes less and less muddy. The upper part remains more or less 

 turbid for a considerable time, since the final velocity of the finest 

 grains is only about 1 inch per day. The result is that the sub- 

 sided mud is moderately firm, but far from homogeneous, being 

 possibly a sort of very fine sand at the bottom, and the finest 

 possible clay at the top. This may be looked upon as a normal 

 layer for one period of mud. On the contrary, if the amount of 

 solid material is decidedly more than 1 per cent, of the volume, 

 the grains collect together into small compound masses, which 

 subside with a small velocity, quite unlike that of the larger or 

 smaller constituent grains, forming an almost, liquid mud, and 

 leaving the supernatant water almost clear from the beginning. 

 This mud slowly decreases in volnme by forcing out the entangled 

 water, but may remain in a semi-liquid state for a long time, which 

 explains a number of interesting facts. The grains of varying 

 size may thus be very little separated, and an almost homogeneous 

 deposit formed, with a mere trace of division into layers ; though, 

 on final consolidation, it may show fissility analogous to imperfect 

 slaty cleavage. 



The following tables (IV & V, pp. 192 & 193) show the character 

 and rate of deposition in the case of the less and the more muddy 

 water during the first two hours and the first eight days. The 

 intermediate state is best shown by an experiment in which the 

 proportion of mud was larger, described later. 



It will be seen from Table lY that, when the mud collects at the 

 bottom as separate grains, it is at first in a somewhat loose condi- 

 tion and afterwards settles down ; but, if the amount were consider- 

 able, it might remain for many weeks in a semi-fiuid condition. 



In an experiment in which the percentage-volume of the dry 

 mud was 15|, semi-liquid mud continued to subside for about 



Q. J. G. S. yo. 254. 



