382 Reports <& Proceedings — Geological Society of London. 



minerals of late Archaean, that is, late Middle Pre-Cambrian, age in 

 Scandinavia (Moss 0-12 to (Mo, Arendal 0-16 to 0-18, and Ytterby 

 0'15 to 0-17), Canada (Villeneuve, Quebec, 0-17), and India 

 (Singar 0-14). 



The rocks are described in detail, with tables giving the 

 quantitative mineral composition and the specific gravities and 

 radium contents. Numerous examples of contact phenomena between 

 crystalline limestones and various types of igneous rock are recorded : 

 pyroxene, amphibole, sphene, and soda-lime felspar being the new 

 minerals chiefly developed, between granite and limestone, with 

 garnet and scapolite in special cases. 



With reference to the origin of the ciystalline limestones and 

 gneisses the following conclusions are arrived at : — 



(a) The crystalline schists and limestones are interpreted as arenaceous 



and calcareous facies of an ancient sedimentary series, their 

 argillaceous complements being unrepresented unless they enter 

 into the composition of the biotite-gneisses. 



(b) The limestones have controlled the formation of hornblende-gneiss 



and amphibolite by their interaction with a granitic magma that 

 elsewhere is represented by biotite-gneisses. The cores of the 

 limestones have been enabled to resist further silicification by being 

 thus enclosed within a blanket of rocks impoverished in silica. 



(c) If the ancient sedimentary series included argillaceous formations, 



it is thought probable that the gneisses are composite rocks 

 produced by the concordant injection of granitic magma into 

 such formations. This view, although not proved, is supported 

 by mineralogical and radio-active evidence, and by the fact that 

 in certain inselberg peaks the banding of the gneisses gradually 

 dies away as the slopes are ascended, the rocks passing into 

 granulitic granite nearly free from biotite and showing few traces 

 of foliation. These peaks are interpreted as the irruptive foci of 

 granulitic magmas which fed the lateral intrusions represented by 

 the surrounding gneisses. 



It is shown that there are at least three types of inselberg peaks 

 that owe their survival to peculiarities of structure and composition. 

 The first type is that just mentioned, in which the foliation is less 

 marked and the biotite content appreciably lower than in the 

 surrounding gneisses. In the second, the peaks are mainly composed 

 of granulitic granite (again poor in biotite compared with the 

 gneisses), and in the third type the peaks are riddled with tongues 

 and apophyses of pegmatite and aplite. In each case the greater 

 resistance offered to denudation is related to the presence of less 

 foliated and more felsic rocks than are found in the adjacent plains. 

 There remains a fourth type — perhaps the most abundant — in which 

 no differences have been recognized. Many of these seem to be 

 isolated relics of gneissic escarpments ; and it is suggested that desert 

 erosion, involving the attack of slopes at their base by arid 

 weathering, and the removal of disintegrated material by wind, is 

 the most favourable condition for the development and maintenance 

 of an inselberg landscape. Existing conditions of denudation are 

 considered to be unfavourable to inselberg survival ; for the peaks 

 appear to be worn down by the removal of superficial layers by ex- 

 foliation more rapidly than the surface of the plateau is lowered. 



