Morgan. — Water in Rock-ma gnxis. 403 



Pegmatites, etc. 



Highly heated water escaping into fissures from a solidifying granite 

 will naturally carry silica and silicates with it in solution. These will ulti- 

 mately crystallize as pegmatite veins (6, pp. 294-96). The consolidation 

 temperature may be below 365° C, and is with tolerable certainty below 

 575° C. (8, p. 342). No difficulty need be experienced in accounting for the 

 various rare or peculiar minerals found in some pegmatite veins. These are 

 generally due to the presence of volatile bodies rejected with water from 

 the consolidating granite. Any special pneumatolytic hypothesis seems un- 

 necessary. In New Zealand pegmatitic veins, as a whole, are remarkably 

 poor in accessory minerals, tourmaline being the only one at all frequently 

 observed. Hence arises one reason why the writer lays stress on water, and 

 not on other volatile substances, as a " mineralizing agent." 



Syenites. 



Syenitic rocks are quantitatively of no great importance, but present a 

 great variety of types. Since they are usually found on the outskirts of 

 granitic masses, it may be assumed that, as a rule, they represent differ- 

 entiates from consolidating granite magmas. Their variety is perhaps due 

 to varying conditions of temperature and of assimilation of solid rock, and 

 more especially to variation in water and other mineralizers present. Some 

 syenites may be derived from igueo-aqueous magmas. 



DiORITES. 



Diorites, more especially the acid types, may in part be due to differen- 

 tiation from a granitic magma. Some diorites may represent consolidated 

 igneo-aqueous magmas, whilst others may be derived from fusion magmas. 



Acid Hypabyssal Rocks. 



The acid intrusives may be regarded as apophyses from granitic magmas. 

 Water probably plays a prominent part in their formation, its presence 

 reducing viscosity and preventing premature consolidation. Quartz- 

 porphyry is said to consolidate, like granite, at temperatures above 575° C 

 (8, p. 342). 



Formation of Schists. 



Where schistose rocks occur in highly folded mountain-chains, and, 

 moreover, are associated w;th granitic masses, -t is easy to account for their 

 format on by invoking a theory of dynamo-thermo-metamorphism. Gently 

 folded schists extending over a wide area, such as the quartz-mica-schists 

 of Central Otago, present a more difficult problem. It has been suggested 

 that the Otago schists are due to thermal nietamorph'sm, induced by under- 

 lying granite. Such a theory implies the formation of igneous rock beneath 

 a wide area. In Central Otago evidence in favour of such a view is afforded 

 by the occurrence in the schists of quartz veins carrying the tungsten mineral 

 scheelite. 



Volcanic Action. 



It is commonly held that steam-pressure has much to do with volcanic 

 eruptions (4, pp. 42, 45). Other imprisoned gases assist, and Harker 

 suggests that gravitational pressure alone may cause fissure eruptions. 

 Arrhenius supposes that sea-water may penetrate by means of fissures to a 



