September 1, 1891.] 



KNOWLEDGE 



165 



ordinary temperature of the air, the appearance of the 

 inclusions is very different. Thus silicate of iron, which 

 frequently crystallizes out from the molten slag of copper 

 works, shows ,'//"-«■.!/ inclusions with xtntidiuiri/ bubbles. 

 Sometimes there is more than one bubble, which could 

 not be the case with a liquid inclusion. In the case of 

 a glassy inclusion with a bubble, the magma has cooled 

 and contracted, leaving a bubble, and solidification has 

 ultimately taken place, whereby the bubble has become 

 fixed. If, however, the whole crystal be heated the 

 included slag melts, the bubble becomes movable, and 

 generally disappears when the temperature has been 

 raised to that at which crystallization took place, the 

 liquid having now expanded so as to occupy the whole 

 of the cavity. 



3. Crystals of other substances show liotJi glassy inclu- 

 sions and inclusions containing water. In such cases the 

 temperature of formation was high, since the now solid 

 magma was then liquid, and the crystallization must have 

 taken place under great pressure, otherwise the water 

 found in the liquid inclusions would have been in the 

 state of vapour. 



i. Lastly, in crystals formed by sublimation (/.c where 

 the substances change at once from the gaseous to the 

 solid state without passmg through the intermediate 

 condition of a liquid), the inclusions are stationary gas 

 bubbles bounded by the actual substance of the crystal 

 itself. 



The type of inclusion serves therefore to show by what 

 process the crystal was formed. The processes obtaining 

 in the formation of the crystals occurring in eruptive rocks 

 and in mineral veins are — 1. Formation from aqueous 

 solution. 2. From igneous fusion. 3. The last two com- 

 bined, which can only happen under great pressure. 

 4. Formation by sublimation. In studying the inclusions 

 in the minerals of Meteorites, Dr. Sorby found glassy 

 inclusions (nih/, neither liquid nor gaseous inclusions 

 being observed. This would appear to show that the 

 Meteorites examined were formed by the process of 

 igneous fusion pure and simple. This conclusion was 

 confirmed in 186G by Daubree {Oedlmjii' K.fjwriiiiriitiilc). 

 His reproduction by the method of igneous fusion ■ of rocks 

 showing many of the characteristic minerals of Meteorites 

 has been referred to in a former article (Knowledge, Jul;/, 

 1891). The brecciated structure of Meteorites, indicative 

 of violent mechanical disturbance or of the agglomeration 

 of heterogeneous fragments, was of course not shown by 

 the products of the crucible. It is principally in such 

 peculiarities of structure and of mode of aggregation of 

 minerals that Meteorites differ from some of the more 

 basic eruptive rocks, such as the diamond-bearing rock of 

 Kimberley. Most nearly approximating to the sidcrolites 

 and aerolites are the so-called volcanic bombs which are 

 floated up with the liquid lava in eruptions, probably from 

 great depths below the earth's surface. 



Till recent years native iron was believed to 

 occur only in Meteorites, and the constant asso- 

 ciation of the metal with nickel was regarded as 

 another peculiarity of these bodies. it has, 

 however, now been shown pretty conclusively " 



that the native iron, found in large masses by 

 Nordenskiold in the Island of Disco, are of telluric 

 origin, having in all probability been left by the 

 weathering away of the basalt which occurs in the 

 locality. This basalt, which belongs to the class 

 of the more basic rocks, is found to contain 

 nodules or balls of iron, sometimes nearly three- 

 quarters of an inch in diameter. This iron (as in 

 Meteorites) is alloyed with nickel, and shows the 



Widmanstiltten figures. It is still undecided whether the 

 basalt contained the metallic matter when in the molten 

 condition before eruption, or whether the presence of the 

 free metal is due to the reduction of silicate of iron by 

 passage through beds containing carbonaceous matter 

 such as are found in the vicinity. 



According to a theory which is advocated by more than 

 one eminent astronomer. Meteorites are Immbs of 

 ancient terrestrial volcanoes, shot into space in bygone 

 ages, and revolving round the sun in orbits intersecting 

 that of the earth, and hence one by one encountering the 

 eai'th, of which they thus become once more a part. This 

 theory of the origin of Meteorites has many points of 

 advantage over those which refer them to the action of 

 volcanoes on other members of the solar system. One 

 point only will be mentioned here — namely, that of bodies 

 shot out from any other planet, only an exceedingly small 

 proportion would intersect the orbit of the earth, whereas 

 bombs from a terrestrial volcano would nil intersect this 

 path. 



Notwithstanding the ability with which this theory has 

 been advocated, the view is still generally held that 

 Meteorites come from the further realms of space, moving 

 in cometary orbits, either parabolic or elongated ellipses. 

 It is true that trustw'orthy determinations are wanting of 

 the velocity of Meteorites in the highest regions of the 

 atmosphere, determinations which would probably furnish 

 conclusive e\idence on this point ; but the paths which 

 Meteorites follow appear to be cometary rather than 

 planetary, in that they do not show any delinite relation 

 to the ecliptic. Several cases have been known of the fall 

 of Meteorites during a shower of shooting stars, though 

 they have not been satisfactorily traced to the radiant of 

 the shower. 



The question of ihe existence elsewhere than on our own 

 planet of the conditions necessary to life, as life is known 

 to us, is a subject which cannot fail to excite interest. 

 The mineralogical examination of ISIeteorites has confirmed 

 the evidence of the spectroscope, that carbon, the element 

 which appears essential to life on the earth, is present else- 

 where than on our o\\n planet. 



SWIMMING ANIMALS. 



By R. Lydekker, B.A.Cantab. 



{I'ontinitcil from jKK/r 154). 



WITH the Fishes, which, with the exception of 

 the ^Vllale, are perhaps, of all animals, the 

 most beautifully adapted for rapid motion 

 through the water, we enter the great group 

 of \'ertebrates. The contour of an ordinary 

 fish, Jsuch as the Perch (Fig. 3), is modelled on those 

 lines suited for cleaving the water best, through which 



I HI I ,.MM..N 1' 



