﻿Vol. 64.] QUANTITATIVE METHODS TO THE STUDY OF ROCKS. 189 



SO far as the facts are known, deposition at something like the rate of a 

 quarter to half an inch per minute may be looked upon as a common average. 

 4. What T have called drift-bed ding in numerous published papers is 

 when the sand is drifted along the bottom to a point where the depth is so much 

 greater and the velocity of tlie current so reduced that it is thrown down at 

 the angle of rest. The velocity of the current is indicated by the nature of 

 the sand ; and probably further experiments would enable us to learn the 

 approximate depth, which probably was small, since an increase of a very few 

 feet made so great a difference in the strength of the current. Excellent 

 examples of this structure are common in many rocks, and the direction and 

 character of the current are sometimes found to have been very uniform over 

 a wide area. 



Some examples of what has been called ' false - bedding ' are 

 irregular accumulations from which no accurate conclusions can 

 be deduced. 



VIII. Joints of Encrinites, etc. 



Each plate and spine of echinoderm and each joint of encrinite 

 is, as it were, a single crystal of calcite, having a complicated 

 external and internal organic structure. The minute, twisting, 

 hollow, internal spaces of big spines are full of air in dry specimens, 

 expelled and replaced by water on boiling, and must be full of sea- 

 water when the animal is alive. It is no doubt by this that the 

 carbonate of lime is introduced when these joints, etc. become almost 

 or quite solid on fossilization. The structure of the test of all species 

 that I have examined, and of the joints of the living Pentacrinus 

 is practically the same, so far as the cavities are concerned. In 

 the case of a big spine of Echinus, I found that the hollow spaces 

 amounted to 51 per cent, of the volume, and the specific gravity 

 when dry was 1'32, and when full of water 1'83 : hence, the excess 

 of weight over water is less than one half of that of a solid shell of 

 the same bulk. These facts fully explain the very special characters 

 of fossilized echinoderms. They do not decay, but are filled with 

 infiltrated calcite in crystalline continuity with the original, so that 

 the structure is similar to that of, a single crystal with the usual 

 cleavage. The specific gravity being so small and their form so 

 very favourable, joints of encrinites would be washed along by a 

 current which would not move fragments of more solid shells and 

 corals of similar size, the specific gravity of which is from 2*7 to 2-8. 

 We can thus easily understand why they so often occur almost or 

 quite free from other material. The same general principles would, 

 to some extent, apply to foraminifera and small univalves. Separate 

 valves of bivalve shells, on the contrary, easily turn over and lie 

 with their convex side upwards, so as to ofi'er much resistance to a 

 current, and may thus be sorted by being left alone. 



IX. Vert Eine-Grained Deposits. 



The properties of extremely -minute particles of clay and chalk 

 diff'er in some remarkable particulars from those of sand, as though 

 a thin adherent film of water played a most important part, when 



