THE DEPOSITS OF THE SEA-BOTTOM. 



57 



of the west coast of Greenland, where it is found in 15 per ct, and a single piece in no. 21 south of 

 Greenland. 



The double refracting grains are of course found in the largest quantity in the not volcanic 

 specimens; but as many of them are also of volcanic origin, as a whole they give no clear idea of 

 the distribution. The same applies also to the class that has been called the opaque double refracting 

 grains; they may belong to every possible mineral, and the border between them and the clear ones 

 is of course very indeterminate, as a mineral grain may well contain smaller inclosings of something 

 without having to be put down as opaque; as, however, I have everywhere put the limit at the same 

 point, I think that the division may have practical validity. 



The most important one of the double refracting minerals is the quartz, but as it is not al- 

 ways to be distinguished from the feldspar, I have joined these two minerals together with a few 

 other indeterminable ones, the quantity of which, however, is always small compared with that of the 

 quartz, to one class. Between the feldspars the plagioclases and the microcline may generally be 

 distinguished from the quartz by means of the twinning striation; sometimes, however, they may 

 be turned in such a way as not to show this; these two minerals are, however, always enumerated 

 separately. It is very difficult to distinguish between the different sorts of plagioclase in grain-pre- 

 parations; for this purpose may be used either the refraction or the directions of extinction. By the 

 refraction is to be noted that the refraction of the albite is slighter than that of the Canada balsam, 

 while the anorthite and most of the mixed forms have a stronger refraction; but' as we may take it 

 for granted that the albite will be very rare in the specimens, and as the measuring of the refraction 

 would necessitate the putting in of a stronger ocular for each grain, I have not thought it 

 worth the while to waste time on such a determination in each single case, so much the less, as we 

 have no absolute warrant that the Canada balsam has always the same index of refraction. The 

 angle of extinction can only in a very few cases be of practical use for the distinguishing between 

 the plagioclases rich in lime, and those rich in soda, and it can at most only be used to distinguish 

 those richest in lime. If we see a grain in such a direction, that the twinning striation is seen plainly 

 and distinctly, that is to say, in a direction ^L (010), we may draw the conclusion that, if the angle of 

 the extinction with the striation be more than 30 , we have a pure anorthite; if it be more than 20°, 

 we have either bytownite or anorthite, and if it be over 5 , the mineral is either labradorite, bytownite 

 or anorthite; if, on the other hand, it be smaller than 5 , we cannot infer that it is one more rich in 

 soda, as the others may well be turned in such a way, that the angle is smaller than the mentioned 

 numbers. The orthoclase may as a rule also be distinguished from the quartz by its being less re- 

 fracting than the Canada balsam, while the quartz is more refracting; but from the reasons set forth 

 above, I have not made this examination, so much the less, as the plagioclases richer in lime without 

 visible twinning striation cannot in this way be distinguished from the quartz. In a few cases the 

 feldspar makes itself distinctly known by being bordered by cleavage planes or by being provided with 

 interpositions arranged after right lines; but as this refers only to part of the grains of feldspar, I 

 have put no stress on these instances. On the other hand, feldspar and quartz, taken as a whole, are 

 easily distinguished from almost all the other minerals found in the specimens, on account of their 

 slight refraction, which makes them appear very indistinctly bordered from the Canada balsam while 



The Ingolf-Expedition. I. 3. o 



