3i2 SCIENCE PROGRESS 



enabling the magma to remain liquid at a sufficiently low 

 temperature to admit of these minerals crystallising out. 1 



Recent researches have thrown considerable light on the 

 question of the temperature of the crystallisation of igneous 

 magmas. It is obvious that no mineral can have crystallised 

 out at a temperature and under conditions in which it is 

 incapable of a stable existence. The " low temperature 

 minerals," for instance, have been employed to fix a higher 

 limit to the temperature of the consolidation of the rocks in 

 which they are found. Quartz is stable only below 8oo° C, at 

 which temperature it tends to pass into tridymite, and it is 

 claimed that no rock containing the former mineral, at least as 

 an original constituent, can have been formed above that tem- 

 perature. 2 The temperatures at which the amphibole and 

 mica become unstable are not exactly known, but they are 

 probably at least as low. 



In considering the value of these arguments it must, however, 

 be remembered that our experimental data were obtained 

 under atmospheric pressure. As the density of quartz (2*65) 

 is considerably greater than that of tridymite (2*3), it is prob- 

 able that under increased pressure quartz would remain stable 

 at a higher temperature. Again, the instability of the amphi- 

 boles and micas at elevated temperatures may be attributed 

 to the fact that they contain an appreciable amount of water. 

 They have apparently crystallised under great pressure in a 

 magma rich in volatile fluxes, and in many cases, it would seem, 

 at a far higher temperature than that at which they become 

 unstable at ordinary pressures for the dark borders seen in 

 porphyritic crystals in volcanic rocks indicate, in all probability, 

 decomposition due to the loss of volatile constituents on release 

 of pressure when the lava was erupted. It would seem that 

 some, at least, of these " low temperature minerals " might with 

 at least equal propriety be termed " high pressure minerals." 



An upper limit for the consolidation of a rock is also fixed by 

 the melting point of its most fusible constituent mineral. The 



1 Morozewicz has shown that the presence of tungstic acid operates in the 

 same manner as the volatile fluxes, the addition of one per cent, to a simple fused 

 magma enabling orthoclase (sanidine), quartz and biotite to crystallise out. This 

 amount is, however, rarely present in igneous magmas. 



2 Wright and Larsen have employed the change of the crystallographic class of 

 quartz at 575°C. to show that granites consolidate above that temperature. Am. 

 Journ. Sci. 1909, 27, 421-47. 



