NO. 14.] MECHANICAL COMPOSITION OF THE SAMPLES. 19 



degree as regards the clayey matter, if no attempt be made to get rid of it. 

 This is best done by filtration. The sample is first thoroughly boiled in 

 order to loosen the clay as much as possible and the liquid filtered, 

 until it is impossible to demonstrate the presence of salt in it. When the 

 boiling has been repeated, the clay will be ready for washing. It frequently 

 proves, however, that even after this treatment the very small particles of 

 the clay coagulate and fall rapidly to the bottom. In this respect there may 

 often be a striking difference between samples that are altogether similar in 

 appearance, and have been treated in exactly the same manner. In the one 

 the clay may be quite finely distributed, and come among the constituent par- 

 ticles 'that are less than - 01 mm.; in the other only a comparatively small 

 amount will remain suspended at this rate of washing, while a large quantity 

 will come among the coarser products. The rule in the main holds good 

 that in those samples containing the greatest amount of sandy particles, the 

 greater part of the clayey matter will also be thrown down, during washing, 

 among the coarser particles. This again is a natural consequence of the 

 circumstance that in places where sand is deposited, the wave-action, or the 

 currents, must have such force that they can remove the finest particles, so 

 that only such of the clay can be deposited as is more coherent than the 

 remainder, virtually only that which originates from older, compact clay de- 

 posits on land, and which have become so firmly cemented together in the 

 course of time, that they can no longer be disintegrated by natural force. A 

 portion may indeed be comminuted by boiling, and there is consequently 

 always, even in apparently, perfectly pure sand-samples, some small per- 

 centage of particles of less than O'Ol mm., especially as some of the sand 

 particles may have been disintegrated and comminuted during the time they 

 have lain upon the sea-bottom. 



It is stranger when in two samples that are otherwise quite uniform, the 

 clay is altogether different. We may here, for example, look at the curves 

 for the samples Nos. 14 and 15. Both contain more clay than anything else, 

 the clay in both being of the same brown colour, which gives the samples 

 externally the appearance of being exactly similar; but the maximum size of 

 the particles in the clay in No. 14 proves to be only a little less than 001 mm., 

 so that a very large proportion of them are found among the coarser consti- 

 tuents, while the clay in No. 15 is extraordinarily fine, and almost entirely 



