Volcanoes: What They are and What They Teach

by John W. Judd

COMPOSITION OF METEORITES.

The first fact concerning these meteorites, which it is necessary to notice, is that they are composed of the same chemical elements as occur in the earth's crust. No element has yet been found in any meteorite which was not previously known as existing in the earth, and of the sixty-five or seventy known terrestrial elements no less than twenty-two have already been detected in meteorites.

There are, however, a dozen elements which occur in overwhelming proportions in the earth's crust. We shall probably not be going too far in saying that these twelve elements—namely, oxygen, silicon, aluminium, calcium, magnesium, sodium, potassium, iron, carbon, hydrogen, sulphur, and chlorine—make up amongst them not less than 999 out of 1,000 parts of the earth's crust, and that all the other fifty or sixty elements are 80 comparatively rare that they do not constitute when taken altogether more than one part in 1,000 of the rocks of the globe. Now all of these twelve common terrestrial elements occur in meteorites, and the fact that the rarer terrestrial elements have not as yet been found in them will not surprise anyone, who remembers how small is the bulk of all the specimens of these meteorites existing in our museums.

We have hitherto insisted on the points of resemblance in the chemical composition of meteorites and that of the rocks of the globe, but we shall now have to indicate some very important points in which they differ.

While in the rocks composing the earth's crust oxygen forms one-half of their mass, and silicon another quarter, we find that in the meteorites these elements, though present, play a much less important part. The most abundant element in the meteorites is iron; and nickel, chromium, cobalt, manganese, sulphur, and phosphorus, are much more abundant in these extra-terrestrial bodies than they are in the earth's crust.

We have already referred to the remarkable fact that in our earth's crust nearly all the other elementary substances are found combined in the first instance with oxygen, and that most rocks consist of the oxide of silicon combined with the oxides of various metals. But this is by no means the case with the meteorites. In them we find metals like iron, nickel, cobalt, &c., in their uncombined condition, and forming alloys with one another. The same and other metals also occur in combination with carbon, phosphorus, chlorine, and sulphur, and some of the substances thus formed are quite unknown among terrestrial rocks. Compounds of the oxide of silicon with the oxides of the metals such as form the mass of the crust of the globe do occur in meteorites, but they play a much less important part than in the case of the terrestrial rocks.

Among the substances found in meteorites are several which do not exist among the terrestrial rocks—some, indeed, which it seems impossible to conceive of as being formed and preserved under terrestrial conditions. Among these we may mention the phosphide of iron and nickel (Schreibersite), the sulphide of chromium and iron (Daubréelite), the protosulphide of iron (Troilite), the sulphide of calcium (Oldhamite), the protochloride of iron (Lawrencite), and a peculiar form of crystallised silica, called by Professor Maskelyne 'Asmanite.'

DIFFERENT KINDS OF METEORITES.

There are other phenomena exhibited by meteorites which indicate that they must have been formed under conditions very different to those which prevail upon the earth's surface. Thus we find that fused iron and molten slag-like materials have remained entangled with each other, and have not separated as they would do if a great body like the earth were near to exercise the varying force of gravity upon the two classes of substances. Again, meteorites are found to have absorbed many times their bulk of hydrogen gas, and to exhibit peculiarities in their microscopic structure which can probably be only accounted for when we remember that they were formed in the interplanetary spaces, far away from any great attracting body.

But in recent years a number of very important facts have been discovered which may well lead us to devote a closer attention to the composition and structure of meteorites. It has been shown, on the one hand, that some meteorites contain substances precisely similar to those which are sometimes brought from the earth's interior during volcanic outbursts; and, on the other hand, there have been detected, among some of the ejections of volcanoes, bodies which so closely resemble meteorites that they were long mistaken for them. Both kinds of observation seem to point to the conclusion that the earth's interior is composed of similar materials to those which we find in the small planets called meteorites.