36 THE DEPOSITS OF THE SEA-BOTTOM. 



essential degree contribute to the deposition of material on the sea-bottom. Besides black 

 and colourless vesicular glass, there has in a couple of specimens been found a few pieces 

 of a brown-gray, vesicular glass, both specimens being west of Iceland. Of pumice the black 

 variety has been found in 13 specimens, almost exclusively southwest of Iceland; in 4 of 

 these specimens it forms the chief mass of the ingredients. Red-brown pumice has been 

 found in a single specimen, no. 17, southwest of Iceland. Common gray pumice has been 

 found rather commonly; it is upon the whole. met with in 15 specimens, situated on all 

 sides of Iceland; in three of these it forms the chief mass, viz. nos. 69, 73, and 74, all situ- 

 ated closely southwest of Iceland. This distribution can only be accounted for in the way 

 that large masses of pumice must be sent forth from this side of the island, probably 

 through the large rivers emptying out there. As at least some forms of pumice are lighter 

 than the water, they will thus more easily than the other ingredients be carried off from 

 land; but the smaller pieces must comparatively quickly be so much soaked with water, 

 that they go to the bottom, while large pieces may be found everywhere floating on the sea. 

 Feldspar may also partly be counted among the volcanic ingredients, as far as the labradorite forms 

 a chief constituent of the basalt, and is found in it as grains. It is, however, not easy to 

 distinguish it from orthoclase, unless it be by microscopic examination, and as the feldspar 

 is only found in subordinate quantities in the specimens, I have not thought it necessary 

 to point out this distinction in detail, so much the less as plagioclase may also be a con- 

 stituent of granite and gneiss and many other rocks ; the single pieces, in which it has been 

 possible to discern a twinning striation, have been classed as plagioclase, while the others 

 have commonly merely been called feldspar, and then the colour has been given as a farther 

 character. 

 Olivine is an absolutely volcanic ingredient being found as grains in the basalt; it is always easily 

 distinguished in the larger grains; in a few instances it has been found bordered by crystal- 

 line surfaces of the common form. Olivine has been met with in 16 specimens, almost 

 always in small quantities; it is chiefly found in the specimens near Jan Mayen ; but it never 

 forms any essential part of the specimens. 

 Augite is chiefly volcanic forming a chief part of the basalt; it may, however, also be found in other 

 crystalline rocks, but in smaller quantity; it is found in about the same specimens as the 

 olivine, that is to say, on all sides of Iceland, and at Jan Mayen; but it is not so much 

 spread , and is found in far smaller quantities. The reason of this is , that the augite , as 

 also the feldspar, is almost exclusively found in microscopically small grains in the basalt, 

 while the olivine forms larger grains. From this again arises the peculiar circumstance that 

 while the latter mineral far surpasses the others, and especially the augite, in quantity in 

 the coarser ingredients, it is only, as will be seen hereafter, found to a small degree in the 

 finer ones, whereas the augite here is found in immense masses, and also the feldspar is 

 rather frequent. 

 Magnetite is certainly chiefly volcanic, but is, however, also often found in most other rocks; there- 

 fore it is one of the least characteristic of the ingredients; it has only been met with in 



