REPORT ON THE DEEP-SEA DEPOSITS. 
327 
II. MmERAL Substances of Extra-Terrestrial Origin. 
Among the many substances contributing to the formation of deep-sea deposits, there 
are a few of small dimensions which it has not been possible to refer to a terrestrial origin. 
Both on account of their small size and their rarity, they make up only an insignificant 
part of any of the samples of the different types of deep-sea deposits, but on account of 
the extra-terrestrial origin attributed to them, and their peculiar distribution over the 
floor of the ocean, they are exceedingly interesting, have given rise to much discussion, 
and therefore merit a detailed description. Mr. Murray first called attention to certain 
of these particles from the deep-sea deposits in the year 1876,^ and described them as 
cosmic dust, pointing out at the same time that these particles were much more abundant 
in all the deep-water deposits far from land, where accumulation must be relatively slow, 
than in other regions of the ocean’s bed. The detailed characters of these magnetic 
spherules, with illustrations, were given by us in a special paper published in 1883, in 
which were also described the brown-coloured spherules or chondres.^ 
When the magnetic particles are extracted from a marine deposit, in the manner 
described on page 17, and placed under the microscope, it will be found that the great 
majority consist of magnetite derived from eruptive and other rocks. Many of these are 
still attached to silicates or vitreous volcanic matter, which clearly indicate their origin. 
But along with these fragments of magnetite or titanic iron, there are other grains 
equally magnetic which do not present crystalline contours, and do not occur in the form 
of irregular grains ; — it is to these that the name of cosmic dust has been applied. 
They may be divided for the purposes of description into two groups : — first, black 
magnetic spherules, with or without a metallic nucleus ; second, brown-coloured spherules 
resembling chondres, with a crystalline structure. 
(a.) Black Magnetic Spherules. 
These magnetic spherules rarely exceed 0 '2 mm. in diameter. Their black and shin- 
ing surface is formed by a coating which possesses the properties of magnetic iron. 
This coating is absolutely opaque and black in thin splinters, has a metallic lustre, is 
attracted by the magnet, and is soluble with difficulty in acids. There is often at the 
periphery of the spherule a more or less pronounced depression. Such are the general 
external characters, which may be verified by reference to the various figures on PI. 
XXIII., chiefly devoted to a representation of particles believed to have a cosmic 
origin. Fig. 1 shows one of these spherules extracted from the powder of a manganese 
^ Proc. Roy. Soc. Edin., vol. ix. p. 258. 
^ Murray and Renard, “ On the Microscopic Characters of Volcanic Ashes and Cosmic Dust, and their Distribution 
in Deep-Sea Deposits,” Proc. Roy. Soc. Edin., vol. xii. pp. 474-495. 
