8o PEOCEEDINGS OF THE GEOLOGTCIL SOCIETY. 



An explanation of these anomalies does not at first sight appear 

 hopeful ; we may, however, notice : — 



1. That the temperature of consolidation for a mineral out of 

 a magma is not necessarily identical with that of the isolated 

 mineral, as one substance acts as a flux upon another. 



2. That the more anomalous results are presented by the rocks 

 which appear to have cooled rather rapidly. 



3. That the presence or absence of water greatly modifies the 

 circumstances both of fusion and consolidation. 



4. That Prof. Daubree's experiments indicate that pressure and 

 the presence of water are favourable to crystal-building. 



Eor instance, in a granite or a tonalite it is obvious that the 

 quartz has been the last mineral to consolidate, while in quartzi- 

 ferous felsites and porphyrites, in rhyolites and dacites (even in the 

 most glassy varieties), it is not rare to find good-sized grains, even 

 bipyramidal crystals of quartz, among the porphyritic minerals. 

 In regard to this, it seems worthy of remark that, among minerals 

 so occurring in the non-holocrystalline rocks, a distiuction is obser- 

 vable, some being so perfectly developed that they seem as if they 

 had consolidated out of the enclosing magma shortly before it solidi- 

 fied, while others appear to have been subsequently modified ; the 

 latter being more or less cracked, fragmentary, corroded at the 

 exterior, and sometimes bordered by ferruginous and other minerals. 

 These distinctions probably indicate difference of history. In the 

 former case I should regard it possible that the molten matter, 

 during its upward passage, had been arrested for a considerable 

 time in a position where any further fall of temperature was practi- 

 cally prevented, and the contained water was unable to escape : 

 then crystal-building would go on ; possibly the development of 

 quartz might be favoured by an increase of the pressure * from the 

 masses welling up behind. When the resistance in front is over- 

 come, the fiuid mass passes upwards and outwards, its temperature 

 falling and its water escaping, so that further crystallization is 

 impeded, and the mass assumes a hyaline or, at any rate, hemicrys- 

 talline condition. 



The fracture of included minerals may be explained bj' strains 

 set up during the motion of the enclosing magma as it approaches the 

 condition of a solid body, while the exterior corrosion probably indi- 

 cates that some local rise of temperature, or increase either in pres- 

 sure or in the quantity of water, has affected the stability of the 

 molecules in the crystal. It must be remembered that, during the 

 intermittent upward progress of a lava-stream, its outer parts, by 

 contact with cooler rock, may at times lose enough heat to allow 

 of the formation of crystals during a pause (for I think that the 

 constant shearing of the molecules in a moving mass would be 

 unfavourable to the development of crystals of any size) ; but that 

 when the mass again moves onward, the more solid crystaUiferous 

 crust may be carried into the interior of the mass, where the tem- 

 perature has remained higher and its environment is diff'erent. It 



* Prof. Daubree's experiments show that pressure and water are Terv 

 favourable to the development of quartz-crystals. 



