THE DEPOSITS OF THE SEA-BOTTOM. 



53 



With regard to the certainty of the determinations of the minerals it has to be noted that the 

 qualities of the minerals do not always appear distinctly in so small sizes of grain as those treated of 

 here, and so it often happens that a grain may resemble one mineral about as much as another, and 

 then it will be a matter of judgment where to refer it. It is also very difficult to draw the limits 

 between the different chief categories, in which the grains were divided; for the present I shall only 

 point it out to be a frequent occurrence, that one part of a grain is not of the same nature as the 

 other; one half part may for inst. be clear, crystalline, double refracting, while the other half is in- 

 transparent; in such cases the grain is referred to the category, that seems to be the larger part of 

 it. Now if the grain consists of an aggregate of several clear, crystalline, and intranspareut individuals 

 the case is still more difficult; if none of the ingredients be decidedly predominant, the grain has to 

 be referred to one of the categories of opaque grains, and as by double refracting, opaque grains I 

 only mean such as show extinction in one direction, and thus belong to one individual, I have always 

 placed such a grain under the very comprehensive category of brown, opaque, single refracting grains, 

 the brown colour being almost always predominant in such cases. Of course we cannot, in itself, be 

 justified in referring an aggregate of double refracting individuals to single refracting bodies, but on 

 a contrary supposition we should have to make a new rubric, which would be rather difficult to keep 

 apart from all the others, and under which only comparatively few cases would fall, and this would 

 scarcely be practical. 



As to the colour of the sand it is principally dependent on the amount of quartz; where this 

 mineral is predominant the sand will get the light, brownish gray colour commonly known from 

 arenaceous quartz; if, on the other hand, the volcanic particles be present in the larger amount, the 

 colour will generally be considerably darker; if it approaches the black, this is owing to a consider- 

 able amount of basalt or basaltic glass, while volcanic tufa always imparts to the sand a very homo- 

 geneous, gray or gray-brown colour; this will also be the case, if there be clay in the specimen so 

 hard, that it has not been washed out. A strongly brown or red-brown colour may either be owing 

 to basalt that is disintegrated on the surface, or to quartz coated with limonite, or it may, what is 

 often the case, be owing to Foraminifera that have, at least partly, been transformed into limonite, 

 so that they have not been soluble in diluted hydrochloric acid. A greenish shade in the arenaceous 

 quartz has its origin from a thin coating on the particular grains of some unknown substance, perhaps 

 some compound of protoxide of iron. 



