BY H. I. JENSEN. 599 



less felspathic and more basic type-nosean continued to cool up to 

 the last. Both these minerals are, however, usually idiomorphic 

 in the alkaline trachytes and are generally represented by 

 pseudomorphs. 



Many of these rocks show evidences of having originally 

 contained idiomorphic nepheline which has been resorbed. 

 Katapleiite and, less frequently, cancrinite are met with in some 

 melanocratic trachytes. Nepheline probably exists in very 

 minute amount in the groundmass of many. 



At great depths (in the magmatic reservoirs), it seems pro- 

 bable, the magma tended to consolidate into magnetite, micro- 

 cline microperthite, nepheline, segirine (Si0 2 = 49-5 %). On 

 rising to higher levels new combinations tended to form; the 

 nepheline became albite in part, and the magnetite went to form 

 arfvedsonite or an segirine-augite equivalent to arfvedsonite 

 (Si0 2 4:4-0%) with the rest of the nepheline molecule. 



The addition of the Si0 2 and Na 2 necessary for this trans- 

 formation is best ascribed to the agency of mineralising vapours, 

 especially HF, which brought Si0 2 and Na 2 from portions of 

 the magma, where they were abundant, to places where they 

 were deficient, as by the following equation : — 



magnetite 

 2 NaF + 5 SiF 4 + 1 1 H 2 + FeO.Fe 2 3 riebeckite 



= 22 HF + Na 2 O.Fe 2 3 .4 Si0 2 .FeO.Si0 2 



These vapours would likewise account for nepheline being 

 changed into albite; thus — 



nepheline 

 Na 2 O.Al 2 3 .4 Si0 2 + 2 SiF 4 + 4 H 2 albite 



= 8 HF + Na 2 O.Al 2 3 .6 Si0 2 



The vapours would play an important function as silica-carriers 

 between the magma and the enclosing rock, and in that way tend 

 to increase its acidity as it rose from deep-seated reservoirs to the 

 surface. 



Biotite was also an original constituent of many alkaline 

 trachytes. It is occasionally seen included in the felspar pheno- 



