THE DEPOSITS OF THE SEA-BOTTOM. 47 



Whether such a disengagement of iron and manganese also takes place on the surface of the 

 clay itself, is not easily decided; it would seem that it might as well take place there as on the stones; 

 but the ingredients of the clay, on the other hand, show no trace of such a thing, which would also 

 impart a very dark colour to the clay, while in reality it is very often almost white. Perhaps the 

 Foramiuifera cannot get the coating; but at all events the grains of sand may be supposed to be 

 able to be thus iucrusted. In this case there are two possibilities: either the surface of the clay is 

 really coated with the dark layer, which is again decomposed when covered by other layers; or else 

 the fauna on the sea-bottom will keep the surface of the clay so much in motion, that a quiet depo- 

 sition is impossible; at all events there is found no dark layer on the mineral ingredients in the 

 Globigerina clay itself or in the other bottom-species. That such a layer shields from disintegration 

 may be seen from a piece of gneiss that is quite crumbled away on the lower side, while the upper 

 surface is considerably firmer. I cannot, however, decide, whether this disintegration has taken place 

 on the sea-bottom, or after the stone has been taken up. 



II. The Ingredients 0-5— o*o5 mm . 



We have now to examine more closely the size of grains next to that treated in the preceding 

 section, and which contains the sandy ingredients of the specimens properly so called. In some 

 respects they give a better result than the ingredients before mentioned, especially as they are found 

 in a quantity sufficient for examination in all the specimens, while many of those contained only very 

 small quantities of coarser ingredients or none at all. On the other hand the particles of this size give 

 as good as no informations of the rocks directly, but only of the minerals contained in them; but 

 nevertheless we are fortunately able to distinguish between almost all the volcanic ingredients and 

 the not volcanic. The chief reason, however, to treat this size of grain in a separate chapter is that 

 by its distribution essentially other laws have been ruling than by the distribution of the coarser 

 sizes. While by these latter we must most frequently seek the origin from so adjacent regions as 

 possible, especially often from the sea-bottom itself, and, where this factor is not sufficient, from the 

 floating ice, these circumstances play no particularly prominent part with regard to the finer ingred- 

 ients, where we must pay essential regard to the currents in order to explain the distribution. This 

 distribution proves also here to be much more regular than by the sizes before mentioned; the two 

 ridges especially are of very small importance here. 



By the treating of the sandy ingredients the microscopic examination must, of course, yield 

 the chief assistance. First, however, I note the larger or smaller fineness of the sand as well as its 

 colour which is rather varying. Next a smaller specimen is treated with a common magnet, by 

 which the magnetic ingredients are extracted and valued to an approximate percentage; this percent- 

 age might, of course, be much more exact by a precise weighing and treating with an electromagnet 

 in water or spirits; but I do not think the result would be sufficiently great to make up for the time 

 it would take, so much the less, as the laws of distribution cannot be put down with any great 

 exactness, especially because the quantity of magnetic material must be taken to be comparatively 

 casual. The next step is to examine by the magnifying glass the presence of certain more conspi- 

 cuous ingredients that might easily be absent in the microscopic preparation, that is to say, besides 



