3H SCIENCE PROGRESS 



tion had already commenced, as is often the case with lavas, this 

 furnishes us with a maximum limit, but the porphyritic crystals 

 of intrusive rocks have often been formed in situ, and accordingly 

 give no evidence of the temperature of intrusion. He considers 

 the rare existence of evidence of the melting of the rocks 

 that once enclosed a molten igneous magma to be an indi- 

 cation that the temperature was little in excess of that of 

 consolidation. It must be remembered, however, that the 

 adjoining rocks contain a comparatively small amount of 

 volatile fluxes, so that their fusion point may be much higher 

 than that of a perfectly fluid magma which is rich in these con- 

 stituents. There is little to warrant any definite conclusion on 

 the subject ; yet it is difficult to traverse Brogger's contention 

 that the temperature of intrusion of a deep-seated magma must 

 be greater than that of its apophyses or any volcanic rocks with 

 which they are connected, but it is probable that the difference 

 is comparatively small. 



In estimating the temperature of the crystallisation of rock- 

 forming minerals, the influence of the high pressure that may 

 have prevailed must not be forgotten. If crystallisation be 

 accompanied by contraction, as is the case with all rock- 

 forming minerals, increase of pressure results in an increase 

 of the temperature at which crystallisation takes place. But 

 although every magma or glass having the composition of a 

 simple rock-forming crystal has a larger volume and lower 

 density than its crystalline equivalent, this is not always 

 the case with one representing a mixture of different minerals, 

 for there are some rock glasses which are found to possess a 

 greater density than that of the combination of minerals which 

 would have been formed if the circumstances had been favour- 

 able for crystallisation. The temperature of crystallisation 

 of such a magma will be lowered instead of raised by in- 

 crease of pressure. There is reason, moreover, to believe 

 that as the pressure is increased, the decrease in volume of 

 individual minerals on crystallisation gradually becomes less 

 and ultimately passes into an increase. We may therefore 

 expect that under very high pressures the temperature of 

 crystallisation of all magmas will be indefinitely lowered so 

 that at a sufficient depth no crystallised minerals will exist, 

 even though the temperature of the earth's interior is not so 

 high as has been supposed. It does not follow from this 



