44 0. B. B0GGILD. BOTTOM DEPOSITS. [norw. pol. exp. 



quantity of this mineral found. Sample No. 15 again, contains rather more 

 hornblende than the samples along the Siberian coast, and it may therefore 

 be assumed that the rocks containing hornblende occur in rather greater quan- 

 tities in east Siberia. 



b. The Lighter Particles. The same method of treatment has been 

 employed for these as for the heavier minerals, namely, that of counting 100 

 grains in a preparation, whereby the percentage of the various minerals is 

 given directly. As the separation of quartz from felspar in sand preparations 

 is exceedingly difficult, in some cases even impossible, I first attempted to 

 separate these minerals by treating the sand with hydrofluoric acid. This 

 acts upon both minerals, but in such a manner that the surface of the felspar 

 becomes coated with a layer of insoluble fluoride, while the surface of the quartz 

 particles remains quite clean. This difference only becomes visible when, after the 

 sand has been washed, it is placed in a solution of fuchsine or some other 

 strong dye. The surface of the grains of felspar then becomes coloured, 

 and their number can be determined by counting. The advantage of this 

 method is that all the different felspar minerals are separated from the 

 quartz; but it is not particularly adaptable in practice, as in spite of all 

 possible variations in the process of the experiment, it appeared that some 

 of the grains became very imperfectly coloured, so that all possible transitions 

 were found, and the number per cent could not be counted with very great 

 exactitude. 



I have consequently employed the difference in refraction to determine 

 the quantity of felspar and quartz. While orthoclase, microcline, albite, and 

 the plagioclases rich in soda, have a weaker refraction than Canada balsam, 

 that of quartz, anorthite, and the plagioclases rich in lime, is stronger. By 

 this means only, some of the felspars can be separated; but the last named 

 plagioclases will certainly occur in very small quantities in materials originating 

 from very quartziferous rocks. In most of the grains they will also be 

 recognised by the oscillatory twinning, as there is a greater probability of their 

 lying vertically than horizontally on the slides. I have not found, however, 

 in any one of the samples, a striated grain that had not a weaker refraction 

 than Canada balsam; and we may therefore conclude from this that these 

 plagioclases must be exceedingly rare. 



