The Bocks of Tristan cVAciuiha. 41 



interior of the earth is very clearly brought out in Prof. Vogt's 

 researches, where he shows that the basic and acid rocks of continental 

 types are each characterised by metals, each group having peculiar 

 metals associated with them. These continental types of igneous 

 rock are solidified portions of the earth's interior, that is to say, 

 they are the original fundamental rock that has had no alterations 

 in its constitution brought about by denudation, deposition, or 

 re-melting. The following table given by Prof. Vogt is expressive. 

 It shows the preponderance of certain metals in the two groups 

 of rocks : — 



Acid Igneous Rocks. Basic Igneous Rocks. 



Silicon to a great extent. Silicon to a less extent. 



Potassium and lithium. 



Barium and strontium. 

 Beryllium. Magnesium. 



Aluminium. 



Iron, manganese, nickel, and cobalt. 

 Wolfram, uranium, and molybdenum. Chromium. 

 Tantalum, niobium. Vanadium. 



Cerium, yttrium. 

 Tin, zirkonium, thorium. Titanium. 



Gold, platinum, iridium, osmium. 

 Boron. Phosphorus. 



Sulphur. 

 Fluorine. Chlorine.* 



The teaching of the above list is to show that when the rocks are in 

 earth's crust below the zone of the existence of stratified rock, that 

 there is a magnetic differentiation, and that there is a separation into 

 two groups, acid and basic. Each group attracts to itself certain 

 elements, including a certain proportion of metals. We find, too, 

 that this differentiation goes on side by side, and is not affected by 

 the distance from the earth's centre. Given, therefore, that the 

 crust of the earth is a slaggy, siliceous mass supported on a metallic 

 centre, the teaching of Yogt's work shows that the slaggy material 

 separates out according to some law of the mutual affinities of the 

 elements, rather than the simple action of gravity on the elements 

 composing the rock magma, according to their densities. We enter 

 on a field of research in which volcanoes have no part, and 

 we see that oceanic volcanic islands are not composed of the 

 material of which the normal original basic rocks of the earth's 

 crust consist. 



* Ueber die relative Verbreitung der Elemente, besonders der Schwermetalle, 

 und die Concentration des urspriinglich fein vertheilten Metallgehattes zu 

 Erzlagerstathen, J. H. L. Vogt, " Zeitschrift praktische geologie," 1898, p. 314. 



