4 8 



THE DEPOSITS OF THE SEA-BOTTOM. 



siliceous organisms as sponge spicules and Radiolaria that are not to be treated in this section, espe- 

 cially dark mica and muscovite, and the fine leaves of colourless, volcanic glass that may often be 

 quite like muscovite, and then have to be distinguished from this by their single refraction. Then a 

 preparation of the sand is made in Canada balsam, and for the sake of homogeneousness the finer 

 ingredients are selected by rolling the sand on a piece of paper, when the coarser sand will always 

 be the foremost portion. By this method we may perhaps get a somewhat different ratio between the 

 particular constituents than what we should get from the sand as a whole; but iu the first place it 

 may be said that the coarser grains are not easily distinguished by microscope, as they will easily, 

 especially if the question be of fine-grained rocks, prove intransparent, while the finer grains more 

 frequently contain the particular minerals; and next it will be very difficult to make a preparation 

 with different sizes among each other, as the larger grains must make the preparation very thick, up 

 to I / 2 n,m , and then the finer grains will be placed in more layers one upon the other, so that they 

 cannot be examined at all. The chief reason, however, is that the larger grains must be supposed 

 to be of almost the same nature as the before mentioned particles over o-5 mm , and thus an examination 

 of them would in all essentials be a repetition. On the other hand it will not do neither to lay the 

 principal stress on the still smaller particles under o-05 mm , as they are only with too much difficulty 

 to be distinguished on account of their smallness, by which we are especially prevented from seeing 

 whether they belong to a more or less double refracting mineral, as also the colour is much slighter. 

 When the preparation is finished a hundred grains are counted off from each of two different fields 

 of view from opposite sides without any selection whatever of the ingredients. The reason of two 

 countings being made is that the ingredients may perhaps have been washed out from each other by 

 floating in the melted Canada balsam. By the enumeration the grains are separated into the three 

 categories: intransparent, single refracting, and double refracting, besides the organisms that are not 

 included in the calculation of the percentage. The single refracting ingredients may either be brownish, 

 intransparent grains, the species of which are not to be determined, or a glasslike substance, mostly 

 brown, but also of several other colours or quite colourless; the garnet is also to be included here; 

 whether other single refracting minerals, as for inst. spinel, are also found, cannot be determined; it 

 may be supposed beforehand that they play only a very small part beside the garnet, and so they 

 may at once be incorporated with this. The double refracting ingredients are separated in the opaque 

 that cannot be more particularly determined, and the clear and transparent ones, and of these latter 

 as many as possible are determined and counted; the rest will commonly be made up of quartz and 

 feldspar, which two minerals most frequently cannot be distinguished from each other. Finally a 

 search is made in the part of the preparation that is not enumerated, to ascertain whether here should 

 perhaps be found single grains of other minerals than those included in the calculation of the percentage. 

 In the first of the following tables is first noted the nature and colour of the sand, and next 

 the contents of magnetic material, mica, and colourless, volcanic glass; the presence of the two latter 

 parts is denoted by a +. Then the percentages of the different sorts of ingredients, obtained by the 

 enumeration from the preparation are noted, except the double refracting minerals that are, for the 

 sake of the space, put off to the second table. The presence of the ingredients is besides denoted by 

 a -|-, when they are not found in the mentioned part of the specimen. 



