5« 



THE DEPOSITS OF THE SEA-BOTTOM. 



the others are generally sharply distinguished, and further on account of the slighter double refraction, 

 which is, however, not so sure as the preceding criterion, as it is to a high degree dependent on the 

 thickness of the grain, and its crystallographic position with regard to the axis of the microscope. 



If we subject the distribution of the quartz and feldspar together to a closer examination, we 

 shall see that in a rather broad belt round Iceland the quantity is under 10 per ct: then it rises 

 quickly with the distance from this island, and reaches as well southwest of Iceland as towards Jan 

 Mayen up towards 70 per ct The largest masses, however, are found at the west coast of Greenland 

 where the percentage is as a rule between 70 and 80, sometimes even more. Twinning striated plagio- 

 clase has been found in almost all the specimens, the largest amount on an average in the Green- 

 land ones, largest in no. 36 with 65 per ct. 



It may, perhaps, also be supposed that the plagioclase forms a greater percentage of the Green- 

 land rocks than of the Icelandic ones, although the basalt contains this mineral as its chief ingredient; 

 but most frequently it is found in the basalt in so small grains, that they do not fall under the size 

 of grains treated of here, unless it be together with the other minerals of the basalt; on the other 

 hand the plagioclase is found, although as a subordinate ingredient, still in no very small quantity, 

 in most of the Greenland rocks, and almost everywhere in rather large grains. As to the microcline, 

 it is not so widely spread as the plagioclase; altogether it has been found in 23 specimens, every- 

 where in small quantities, but evidently also most commonly in the Greenland specimens. 



Among the other double refracting minerals, the augite is by far the most conspicuous; it is 

 rather easily distinguished from most of the other minerals except the olivine; as, however, both of 

 these two minerals are chiefly of volcanic origin, a confounding of their identity will be of no great 

 consequence. Their colour is almost the same; the augite is commonly of a peculiar brown-gray, 

 slightly violet colour; more rarely the green form is met with in the specimens; this form, however, 

 is easily distinguished from hornblende by its almost complete want of pleochroism. When the augite 

 is prismatic, what is more rarely the case, it is very easily distinguished from olivine and hornblende 

 by its large angle of extinction; but most frequently the grains, on account of the slight cleavage, 

 are rounded without any conspicous longitudinal direction, and in such cases the refraction has to be 

 taken into consideration; the refraction of augite is a little larger than that of olivine; but the diffe- 

 rence is too small to be used as a characteristic; on the other hand the olivine has a considerably 

 larger double refraction, viz. 0-033 against 0-023, an( ^ by tms ** ma -Y often be distinguished though 

 with some uncertainty. The olivine is commonly almost colourless, sometimes with a slight green 

 tint; nevertheless the colour is no reliable character. The augite is found in the largest quantity in 

 several of the Icelandic specimens; but the percentage is, for the rest, rather deviating from place to 

 place without any special regularity. It is found in especially large quantity in the specimens north 

 of Iceland, where it rises to the absolutely largest percentage, 32%, in specimen 128. It is likewise 

 found in large quantity in the specimens between Iceland and Jan Mayen, while east of Iceland it is 

 less conspicuous. North of the Faroe Islands much augite is found; but on the ridge between these 

 islands and Iceland very little of this mineral is met with. Southeast of Iceland it is found in large 

 quantities, but in the large number of specimens southwest of Iceland it is quite minimal. Between 

 Iceland and Greenland the quantity is very irregular, as is also the case with the specimens from 



