1893-94-] The Interior of the Earth. 105 



we may take a dozen as occurring in conspicuously large pro- 

 portion in the earth's crust, the remainder forming a relatively 

 insignificant part. The twelve elements are oxygen, silicon, 

 aluminium, calcium, magnesium, sodium, potassium, iron, carbon, 

 hydrogen, sulphur, and chlorine. 



The rocks forming the earth's crust are composed half of 

 oxygen and one-fourth of silicon — that is to say, they are 

 mostly made up of. silica or silicates in one form or another. 

 In meteorites, though these compounds are present, they are 

 usually in very small proportion, and the bulk of meteorites is 

 generally metallic, the predominating metal being iron. Many 

 of the meteorites containing some amount of stony material 

 have been found to yield minerals identical with many which 

 occur in rocks of the earth found at great depths or brought 

 up by volcanic agency — namely, olivine, enstatite, augite, anor- 

 thite, magnetite, &c. The minerals thus found in meteorites 

 always resemble those which occur in the more basic volcanic 

 rocks. Quartz, acid felspars, and other minerals which occur 

 in acid rocks are never met with in these bodies of extra- 

 terrestrial origin. Among volcanic products rocks and lavas 

 called ultra-basic are met with in comparatively rare instances, 

 but a remarkable fact about them is that they are almost 

 entirely made of minerals of precisely the same character as 

 those found in the stony portions of meteorites. The Picrites 

 are an example of the ultra-basic volcanic rocks, and I believe 

 an exposure of this rock is still to be seen in the new Barnton 

 railway cutting, if it has not by this time been " dressed " out 

 of sight by the workmen. The specific gravity of these ultra- 

 basic rocks is high, usually above 3, and they contain a low 

 percentage of silica and a comparatively high one of magnesia 

 and iron. Eecently a most interesting discovery was made, 

 namely, that materials very closely resembling the metallic 

 portions of meteorites are sometimes brought to the surface 

 by volcanic means. In the year 1870 Nordenskiold found 

 at Ovifak, on the south side of the island of Disko, off the 

 coast of Greenland, a number of blocks of iron which agreed 

 in composition with ordinary metallic meteorites, as was seen 

 on a chemical examination of specimens. It was at first 

 supposed that a lot of meteorites had here curiously fallen at 

 one spot. A detailed examination of the district by Pro- 



