BY H. I. JENSEN. 



585 



3. They occur only in regions where the dominant displacements 

 were vertical, that is where folding played little part. 



4. Very old rocks, often the Archaean complex, occur in or very 

 near every alkaline province. 



5. Almost all the great alkaline eruptions of all parts of the 

 earth took place in the Eocene period. 



6. It is suggested that alkaline rocks are only exceptionally 

 produced by differentiation from a normal magma. They are in 



Fig. 10. — The Australian Trachyte Line, or Line of Outcrop of 

 Alkaline Rocks. 



general produced from an alkaline magma, and such a magma is 

 not formed in the same way as a normal magma. 



7. For reasons given the theory is advanced that alkaline rocks 

 are derived from, in the first place, Archaean saline beds which, 

 by chemical attacks on the adjacent sediments, have given rise 

 to an alkaline magma in the process of metamorphosis. This 

 magma has been squeezed laterally into the continental areas, 



43 



