ANALCITE GROUP OF IGNEOUS ROCKS 687 



presence of water in undoubted pitchstone glasses as a proof 

 that water may be retained by igneous magmas at high tempera- 

 tures. We believe this explanation to be the correct one and 

 will present some further proofs of its probability. 



The intimate relation between biotite on the one hand and 

 olivine and leucite or orthoclase on the other was pointed out by 

 Iddings' and has been further discussed by Backstrom^ and the 

 author. 3 Iddings and Backstrom point out that since for the 

 production of biotite certain mineralizing agents, such as water 

 and fluorine, are necessary, since they enter into its composition^ 

 the biotite rich rocks must be intrusive ones, whereas if the 

 magma attains the surface and under the diminished pressure 

 the water escapes, then olivine and leucite may be produced. 

 By this is explained the general absence of leucite in abyssal 

 rocks and its frequency in extrusive lavas. This process would 

 of course find its most natural expression in magmas rich in mag- 

 nesia and potash. 



When we consider the magmas in which soda predominates 

 however, it is clear that quite different processes will take place. 

 There is no such relation between soda and magnesia as is shown 

 by potash and magnesia in the biotite molecule. Therefore we 

 might expect that if the magma contained water vapor and soda 

 predominated in it, that analcite would be formed if the magma 

 crystallized under pressure with considerable rapidity, whereas 

 if the magma were anhydrous or the water vapor could escape 

 without taking part in the crystallization either by relief of pres- 

 sure or b}- ver}' slow and gradual processes of crystallization, 

 which would exclude it, then we should expect nephelite to form 

 or nephelite and the albite molecule, the latter, perhaps, giving 

 rise to plagioclase. 



From this it would follow that the conditions most favorable 

 for the production of primary analcitic rocks would be in dikes 

 and small intrusions which is in fact the place where they occur, 



'Origin Igneous Rocks, Bull. Phil. Soc, Washington, Vol. XII, p. 176, 1892. 

 *Geol. Foren. Forh., Stockholm, Vol. XVIII, p. 161, seq., 1896. 

 3High\vood Mts. Bull. Geol. Soc. Am., Vol. VI, p. 409, 1895. 



