508 Dr. Shand — Saturated & Unsaturated Igneous Rocks. 



likewise riclier in lime ami poorer in alumina than felspar. The 

 vesuvianite aggregates, therefore, are the product of the solution of 

 hornblende crystals in a fluid so viscous that but little diffusion 

 could take place. The interstitial vesuvianite arises through a com- 

 bination of two causes : (1) The ratio of lime to alumina in the 

 diorite is, as stated above, higher than that required for the formation 

 of epidote. (2) The diorites contain appreciable amounts of magnesia 

 and alkalis. Part of the former crystallizes as spinel, but the 

 remainder, together with the alkalis, would enter the vesuvianite 

 molecule, which can have up to 6 per cent magnesia and 2 per cent 

 alkalis, while epidote rarely has more than a trace of either. 

 (To be concluded in our next Number.) 



V. — On Saturated and TJnsatueated Igneous Rocks. 



By Professor S. J. Shand, D.Sc, Ph.D., F.G.S., Victoria College, 

 Stellenbosch, South Africa. 



OF the various minerals which enter into the composition of igneous 

 rocks, about one-half are capable of forming in presence of free 

 silica, as is shown by their association in rocks with quartz and 

 tridymite. These may, for the present purpose, be termed saturated 

 minerals ; they include some of the most highly silicated compounds 

 of their respective metallic elements. The remaining rock minerals 

 do not appear in association with free silica, and may be presumed to 

 be incapable of stable existence in its presence; they are mostly the 

 less highly silicated compounds of the same metallic elements which 

 enter into the former group. These may, for the present purpose, be 

 termed unsaturated minerals. The unsaturated character of any 

 mineral in the latter group is not affected by the temperature and 

 pressure at which crystallization takes place; this is shown by the 

 fact that quartz and the minerals referred to are mutually exclusive 

 both in plutouic and in effusive rocks. In the case of sodium, 

 potassium, calcium, and magnesium the formation of saturated or 

 unsaturated minerals appears to depend only on the amount of silica 

 available in the magma. Aluminium, ferrosum, and ferricum, elements 

 of feebly basic character, have their combinations largelv determined 

 for them by the more strongly basic elements, with which they tend 

 to form either complex molecules or mix-crystals. 



In the following table the saturated and unsaturated compounds of 

 each element are placed as far as possible in apposition : — 



Saturated. Unsaturated. 



Orthoclase. Leucite. 



Albite. Nephelite. 



Socialite. 



Nosean. 



Analcite. 



Cancrinite. 

 Anortbite. Hauyne. 



Melanite. 



Melilite. 

 Pyroxenes, "v Olivine. 



Amphiboles. I Pyrope. 



Micas. J Picotite. 



