THE GENESIS OF IGNEOUS ROCKS 313 



temperature of the crystallisation of a dolerite must therefore 

 be below 1170 C., the melting point of augite. As a conse- 

 quence, however, of the eutectic principle the magma must 

 have remained liquid to a still lower temperature, and the 

 determinations of the temperature of the molten lavas of 

 Vesuvius, which place it in the neighbourhood of iooo C., 

 agree very well with theoretical considerations. 



The presence of a large amount of volatile fluxes will also 

 diminish considerably the temperature of solidification, and we 

 may accordingly expect it to be lower in deep-seated than in 

 volcanic rocks, which have lost the greater part of these con- 

 stituents. Some pitchstone dykes yield water to the extent 

 of over 20 per cent, by volume, and it is pro"bable that a granite 

 magma contains even more. Sorby as long ago as 1858 showed 

 how the temperature of the crystallisation of a rock may be 

 calculated from the proportion which the volume of the liquid 

 inclusions in quartz and other minerals bear to the cavities in 

 which they occur. In this way it has been found that the 

 temperature of crystallisation of the Cornish granites lay 

 between 200 C. and 350 C. (But see note 2 on p. 312.) The 

 data obtained for the associated quartz-porphyry dykes indi- 

 cates, Mr. Harker believes, a temperature some 50° higher, the 

 result no doubt of a smaller amount of volatile fluxes. 



The pegmatoid veins, which are also connected with granite, 

 are believed, on the other hand, to have crystallised from a 

 magma still richer in the elements of water and other volatile 

 constituents than the granite itself, and they no doubt solidified 

 at a much lower temperature. They may sometimes be 

 followed until they pass into quartz veins which must have 

 been deposited from a magma consisting mainly of silica and 

 water, without alumina and the alkalies, at very moderate 

 depths, and at a temperature little above the boiling point of 

 water under atmospheric pressure. The extreme limit is 

 represented by the waters of geysers that deposit siliceous 

 sinter at the surface as they cool. 1 



Mr. Harker also discusses the question of the temperature 

 of magmas at the time of intrusion underground, or extrusion 

 at the earth's surface. Where there is evidence that crystallisa- 



1 The zeolites are in many cases among the latest deposits from magmatic 

 water. The presence of fluorine in some form is indicated in those cases in 

 which apophyllite occurs. 



