414 



/. H. L. VOGT 



The relation between Or and Ah+An in granitic rocks lies 

 with a few exceptions within the boundaries 0.7 Or : 0.3 Ab+An 

 and 0.2 Or : 0.8 Ab-\-An. In about half of the heretofore pubhshed 

 analyses^ the proportions vary within the rather narrow boundaries, 

 0.5 Or : 0.5 Ab+An and 0.35 Or : 0.65 Ab+An. For a very large 

 number we find almost exactly 0.4 Or : 0.6 A\)-\-An. The other 

 components, besides quartz comprise a small percentage of Mg+Fe 

 -silicates, magnetite, etc. 



A very large number of the granitic rocks, chemically, lie 

 very close to a compUcated eutectic, Qu : Or : Ah -\- An : about i 

 per cent ferromagnesian silicate and about i per cent magnetite, 

 etc. The melting point,^ when the temperature is recalculated to 

 one atmosphere pressure and when H2O, etc., is not taken into 

 consideration, is not higher than about 950°, probably between 

 950° and 900°. 



A small percentage of Mg-f Fe-siHcate, magnetite, etc., above the 

 just mentioned comphcated eutectic, will only raise the beginning 

 of the crystallization a few degrees. A surplus of feldspar, in some 

 cases orthoclase (microcline) , in others a plagioclase rich in soda,— 

 will raise it a small amount, say 50° or thereabouts. Hence, the 

 crystallization in the most common granitic rocks (without a sur- 

 plus of quartz), calculated at i atmosphere pressure and without 

 regard to the H2O, etc., of the magma, will not have begun at a 

 higher temperature than 1000°, or, for rocks especially rich in Or, 

 perhaps as much as 1050°. Most granitic rocks must have been 

 quite fluid even a little below 1000. The conclusion of the crystal- 

 lization will have taken place at a maximum of 950° and probably 

 between 950° and 900°. In rocks with more than 75 or 76 per cent 

 silica, the relatively small amounts of quartz present in surplus 

 above the decisive eutectic, will raise the temperature of the begin- 

 ning of the crystallization a little above the temperature of the 



' As to the analyses published up to a few years ago, I refer to the statistic view — 

 including about 600 analyses of granitic rocks — given in my. treatise, Anchi-mono- 

 mineralische tind anchi-eutektische Eruptivgesteine, 1908, p. 75. 



2 Instead of melting point it should here, where the plagioclase as well as the 

 ferromagnesian silicates are mLxcrystals, be written: "A short distance on a eutectic 

 boundary-line or-plane." 



