DIFFERENTIATION OF IGNEOUS ROCKS 413 



6, 5 or perhaps even as low as 4 or 3 per cent) begin to crystallize 

 at an earlier stage than the feldspar. 



As little as 2 or a very few per cent of ferromagnesian silicate 

 in the eutectic may only insignificantly diminish the temperature 

 of the crystallization (as for instance 10 or 25°). 



The conclusion is that the crystallization of syenites, in most 

 cases containing only 5, 10 or up to 15 per cent of ferromagnesian 

 silicates (presuming i atmosphere pressure and anhydrous melts) 

 may have begun at about 1100° — ^in some cases up to about 1150° — 

 and that the final solidification of the end-magma took place at a 

 temperature a little below 1050°. 



A small, as 2 to 6 or 8, per cent of quartz in quartz-gabbros, 

 -norites, -diorites, or -syenites will bring about only a slight lowering 

 of the temperature at the beginning, but a greater lowering at the 

 close of the crystallization because there finally will result a granitic 

 eutectic. 



The granitic rocks. — -Because the binary eutectic Or : Qu and 

 Ab : Qu contains a considerable quantity (about 26 or 28 per cent) 

 of quartz, we have to deal here with a decided lowering of the 

 melting point, namely, 100° or a little more (as 125°) below the 

 melting point of the given feldspar.' If the effect of pressure is 

 left out of the question, the Or : Qu eutectic must be estimated at 

 about 1100° or 1075°, and the eutectic Ab : Qu at about 1000 

 or 975°. For the ternary eutectic, Or : Ab : Qu, a temperature 

 about 100° (or 125°) lower than for the binary eutectic Or : Ab, 

 must be assumed, consequently about 950 (or 925°). The An con- 

 tained in the plagioclase will occasion some increase in the tempera- 

 ture, but this rise is relatively small, since the plagioclase in the 

 granites (albite-oligoclase, and oligoclase) as a rule only contains 

 a small quantity of An. 



Most granites (with 70-75 per cent SiOz) contain between 20 

 and 28 per cent quartz, hence a Httle less or just about as much 

 quartz as the eutectic boundary -fine between quartz and K-feldspar 

 -j-Na-rich plagioclase. Very rarely are the granitic rocks, including 

 the quartz-porphyries and related rocks, so rich in silica that they 

 contain even a small surplus of quartz above the eutectic. 



' See p. 336 (and Fig. 3) in this Journal for 1921. 



