Dr. Shand — Saturated & Unsaturated Igneous Rocks. 509 



I anticipate that objection may be raised to one or two of the 

 minerals classed here as unsaturated. The first of these is olivine. 

 A few dolerites and basalts have been described as containing both 

 quartz and olivine, but I think the experience of most petrographers 

 will show that this association is quite unusual. I have never come 

 across an instance of it in the field, and I think it highly probable 

 that the explanation of such an unusual association is that the quartz 

 is not of magmatic origin. The only example of a quartz-olivine 

 rock in my collection is the 'melaphyre' of Albersweiler in the 

 Rhenish Palatinate, which contains small ' phenoerysts ' of quartz. 

 An examination of this rock decidedly suggests that the supposed 

 phenoerysts are really xenocrysts. They are just as often aggregates 

 as single grains, and they are always anhedral, with rounded, fretted 

 outlines. But to be sure of this one would require to study the rock 

 in the field. Harker ^ and Mennell ^ have both described occurrences 

 of quartz xenocrysts within basalts and dolerites. In the cases 

 described by Harker, basalt sills which were invaded by granophyre 

 have been acidified and impregnated with quartz xenocrysts by the 

 latter. In the dolerite intrusions in the Matopo granite, which 

 Mennell describes, the quartz xenocrysts and the interstitial 

 micropegraatite of the dolerites are due to absorption of granite. In 

 at least one of these intrusions olivine is present. As the latter is 

 a mineral of very early crystallization it is evident that the absorption 

 or incorporation of quartz may occur too late to prevent the partial 

 separation of magnesium in the form of olivine, and hence the two 

 minerals may exist side by side. According to this view such, 

 quartz-bearing dolerites would be hybrid rocks, not true magmatic 

 products. For further evidence bearing upon this point I must await 

 the judgment of those who have the opportunity of studying such 

 exceptional rocks in the field. 



As regards corundum, I know of no instance where it occurs in 

 association with quartz in igneous rocks ; its common hosts are 

 nephelite-syenites, anorthosites, and peridotites. Any excess of 

 alumina over bases in the quartz-bearing rocks always seems to 

 appear as muscovite or topaz, not as corundum. 



It may seem strange that garnets should fall into two contrasted 

 groups, the spessartites (and probably the almandites as well) being 

 regarded as saturated while others (melanite, pyrope) are unsaturated ; 

 yet this is simply an expression of observed facts of distribution. In 

 my own experience of melanite rocks in Sutherlandshire,^ where 



^ Tertiary Igneous Rocks of Skye (Mem. Geol. Surv., 1904). 

 ^ " Basic Dykes and Eock Genesis " : Geol. Mag., 1911. 

 2 Trans. Edinb. Geol. Soc, 1910. 



