Dr. Shand — Saturated & Unsaturated Igneous Rocks. 511 



first to saturate itself chemically, then to oversaturate itself in the 

 physical way. (lleactions with constituents other than silica are for 

 the moment disregarded.) As a consequence of the greater activity 

 of an nndersaturated magma, one would expect to find greater 

 variation, both chemical and mineralogical, within a body or complex 

 of undersaturated rocks than in a saturated or oversaturated body or 

 complex. This deduction seems to be confirmed by the enormous 

 number of varietal names which have been coined for different facias 

 of the undersaturated rocks, 



3. Towards the aluminous constituents of invaded rocks it would 

 seem that there would be little difference in the behaviour of 

 saturated and undersaturated magmas. The change by which 

 aluminium silicates are converted into micas and felspars, a change 

 for which the field evidence is overwhelming in amount, could not 

 be effected any more readily by a felspathoidal magma than by ■ 

 a felspathic one, since in both felspars and felspathoids the ratio of 

 Na, K : Al.is the same. This particular conversion must be due to 

 an excess of alkali in the magma, of which no trace remains as such 

 when the rock has solidified and become exposed to investigation. 

 With this point, however, I am not at present concerned. There 

 remains the theoretical possibility that aluminium silicates might be 

 reduced to corundum by reaction with an undersaturated magma. 



4. When the invaded rock is a carbonate or other non-silicate 

 rock, or contains much lime, magnesia, or iron in the form of oxide 

 or carbonate, then the advantage as regards absorbing power lies 

 with the saturated and oversaturated magmas, which can yield first 

 their excess of silica, and secondly a further quantity of silica due 

 to the reduction of sodium, potassium, calcium, and magnesium 

 molecules from the saturated to the unsaturated state. In this way 

 a saturated or oversaturated magma may become undersaturated. This 

 case, as regards alkaline rocks, has been presented by R. A. Daly.^ 



The above deductions involve no assumption at all as to the extent 

 to which absorption takes place in nature ; so long as even a few 

 instances arise in which absorption is admitted to have taken place, 

 then it becomes necessary to recognize the essential difference in the 

 absorptive capacity of saturated and undersaturated magmas. The 

 appearance of, let us say, nephelite in a syenitic rock means far more 

 than the mere addition of another name to the list of accessory 

 minerals present in the rock ; it involves a real difference, both of 

 kind and degree, in the reaction of the magma towards its environ- 

 ment. By studying the distribution of the saturated and under- 

 saturated members of an igneous complex one may gain a wider view 

 of the process of differentiation than is to be had from the study of 

 minute mineral and chemical differences alone. 



With these considerations in view, it is instructive to turn to 

 the distribution of oversaturated and undersaturated rocks in the 

 litliosphere. 



1. As regards mere bulk, the oversaturated and saturated rocks 

 predominate enormously over the undersaturated. Daly ^ estimates 



1 " Origin of the Alkaline Eocks " : Bull. Geol. Soc. Amer., 1910. 



