THE DEPOSITS OF THE SEA-BOTTOM. ,c 



Volcanic glass and pumice. The two sorts of volcanic glass that may be found in these de- 

 posits, are only with difficulty to be distinguished from each other: the common form, the 

 basaltic glass or the palagonite, is as a rule intransparent to the naked eye, even in small 

 splinters, while the microscopic grains are commonly of a strong brown colour. The acidic 

 glass or the obsidian, on the other hand, is most commonly transparent with a gray or 

 brown colour in small splinters, while under the microscope it is most often colourless; 

 sometimes the obsidian is as strongly coloured as the palagonite, and then it is hardly to 

 be distinguished from this latter. In the vesicular or pumice-like varieties, the two sorts 

 are as difficult to make out; black or dark brown pumice will probably always be basaltic, 

 as also a brown variety; the gray pumice, on the contrary, may as well be acidic as basic; 

 sometimes gray varieties of pumice are found passing evenly into the black ones; one and 

 the same piece of pumice may even have both colours, and is thus to be regarded as 

 basic, while the most lightly coloured and lightest varieties must be richer in silica. The 

 categories of pumice and vesicular glass will, of course, pass into each other; the latter 

 designation has been used, when the glassy mass appeared to have the predominance of the 

 vesicles. The black palagonite without vesicles is found in 8 of the specimens situated as 

 well to the east as to the west of Iceland, a single one, no. 21, south of Cape Farewell; as 

 it is to be supposed that all varieties of volcanic glass have their origin from Iceland (or 

 Jan Mayen), we must asstime that a very long transport has taken place. Obsidian has, 

 with rather great certainty, been found in two specimens, nos. 21 and 54, the former south 

 of Cape Farewell, the latter southwest of Iceland. Black, vesicular glass is one of the most 

 common volcanic ingredients; it is found in 20 specimens situated on all sides of Iceland; 

 in 6 specimens all situated between Iceland and Jan Mayen, it is the chief ingredient; in 

 the specimens nearest to the latter island, a special variety of the rock is found, which, as 

 before mentioned, is distinguished under the microscope by the very numerous, very small 

 vesicles; this form has been found in very large quantities in the specimens 115 and 116, 

 in a smaller quantity in specimen 117. Colourless, vesicular glass is almost always found 

 in small curved plates that may be rather similar to muscovite ; the vesicles are often strongly 

 prolonged in one and the same direction; this fact has been observed in 12 specimens, 

 distributed round Iceland with the exception of the south side, as also towards Jan Mayen 

 and north of the Faroe Islands; in two specimens it forms the chief constituent, viz. no. 98, 

 west of Iceland, and no. 129, northwest of this island. This latter specimen, which, as before 

 mentioned, forms a natural exception to the rule of the quantity of carbonic acid being 

 dependent on the distance from land, it containing about 50 per ct. of carbonate of lime, 

 although it is closer to the coast of Iceland than any other specimen, forms a fully as 

 remarkable exception with regard to the quantity of coarser ingredients; although situated 

 at a distance of but about 40 kms. from the land it contains only a few small leaves of 

 colourless glass, while we had so much the more reason to expect larger masses of minerals, 

 as the locality in question is frequently passed by icebergs; thus this specimen may be 

 regarded as one of the best proofs of the fact, that the icebergs do not commonly to any 



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