510 Dr. Shaud — Saturated & Unsaturated Igneous Rocks. 



quartz syenites pass insensibly into luelanite syenites, the first trace 

 of melanite does not appear until the last trace of quartz has been 

 lost, although there is no immediate change in total lime content. 



Anorthite, too, appears at first sight to occupy an anomaloiis 

 position; it is an orthosilicate occurring in a group which is mainly 

 composed of polysilicates and metasilicates. But calcium metasilicate 

 (wollastonite) is incapable of independent existence under magraatic 

 conditions, hence the orthosilicate is the stable form even in quartz- 

 bearing rocks. True quartz-anorthite rocks are probably rare, but 

 the anorthite molecule must have been present as such in the parent 

 magma of every quartz-plagioclase rock. 



Magnetite and ilmenite are of course completely unsaturated as 

 regards silica, from the chemical point of view, but as both are 

 capable of stable existence in the presence of free silica under 

 magmatic conditions, they rank here as saturated minerals. (Should it 

 be felt that this new use of a common term is liable to be misunder- 

 stood, the reader may at his pleasure read sated and imsated in place 

 of ' saturated' and ' unsaturated '.) 



A rock Avhich contains only saturated minerals may be termed 

 Si saturated rock; one which contains only unsaturated minerals may 

 be termed an tmsaturated rock. It will also be desirable to distinguish 

 partsatiirated rocks (e.g. phonolites, olivine-dolerites) from wholly 

 unsaturated ones ; as a general term to cover both partsaturation and 

 unsaturation we may employ %mder saturation. Any rock which 

 contains free quartz or tridymite of magmatic origin will be termed 

 oversaturated. 



These distinctions are by no means trivial; they have a very 

 definite chemical significance which has a bearing upon the reaction 

 of the magma towards invaded rock masses, and hence upon magmatic 

 differentiation. The)' are also capable of useful application in the 

 classification of igneous rocks. 



1. An undersaturated magma (i.e. one which on solidifying would 

 give rise to a partsaturated or unsaturated rock) is capable of entering 

 into chemical combination with the silica of invaded rock masses. 

 The reactions thereby induced would be exothermic, and would tend 

 to raise the temperature of the magma. The amount of heat to be 

 gained in this way does not seem to be susceptible of direct measure- 

 ment at the present time : the difficulties to be overcome would be 

 very great, but as silicic acid is known to be an exceedingly powerful 

 acid at high temperatures, it is probable that the heating eff'ect of 

 the reaction would be considerable. The access of heat produced in 

 this way would in turn enable the magma to perform a further 

 amount of work in the way of mechanical solution. 



2. A saturated or oversaturated magma (i.e. one which on solidifying 

 would give rise to a saturated or oversaturated rock) is incapable of 

 combining chemically with silica ; its action on the quartz of invaded 

 rocks must be confined to physical solution, wliich will lower the 

 temperature of tlie magma. 



Other things being equal, then, an undersaturated magma must 

 liave a greater action upon invaded siliceous rocks than a saturated 

 or oversaturated magma ; as a result of such action it would tend 



