Papers read hefore Section C {Geology). 515 



plateau-surfaces, as frequently in the case of detached flat-topped and 

 pyramidal sandstone hills, or by the weathering out of the more 

 resistant units iwhere the surface is composed of rocks of different degrees 

 of hardness, as in the case of escarpments and of detached crags and 

 hills of igneous rock. The kopjes and island mountains of the warmer 

 temperate and tropical regions are essentially of similar origin, but 

 on account of the present climatic conditions being in many cases 

 different from those under which they were formed, their actual mode 

 of origin has given rise to considerable discussion. A striking feature 

 of these kopjes and inselberge is that they rise at intervals from 

 an apparently level or gently undulating plain, which in most, if not 

 all, cases should be regarded as a former base-level of erosion. The 

 typical kopjes of South Africa are of sandstone, shot with veins and 

 dykes of igneous rock, which has given them the necessary power of 

 resistance to the agents of erosion. The old crystalline regions of Africa, 

 however, are dotted with domes and turtlebacks of granite and 

 detached groups of granite hills, which represent the more resistant' 

 elements of the crystalline complex. Some of these isolated hills 

 possess flattened cap's of weathered rook, and it seems probable, there- 

 fore, that the sculpturing of the original crystalline surface was due, 

 not so much to the direct erosion of the unweathered rooks, as to the 

 effect of periods of elevation and erosion following upon periods of 

 decomposition in situ at base-level. As the result of erosion a some- 

 what irregular surface would be produced, but a slight subsequent 

 negative movement would suffice for the obliteration of the minor 

 irregularities and the consequent accentuation of the less weathered 

 portions of the surface. The repetition of such a cycle would lead to 

 the increased prominence of the earlier hillocks and the formation of 

 others of lower level. It has been suggested that a landscape with 

 inselberge is of desert origin, but the various phenomena can be 

 explained more readily as the result of weathering and erosion during 

 successive small oscillatory movements of a regional character in the 

 neighbourhood of base-level. 



VIII. — The Heavy Mineral Grains in the Sands of the Scottish 

 Carboniferous.' By T. 0. Bosworth, B.A., B.Sc, F.G.S. 



THESE observations were made at the commencement of an investiga- 

 tion which the author has now no opportunity to carry on. 

 The chief heavy grains are : — ■ 



Garnet, which, though in some sands practically absent, in others 

 is in such excessive quantity as to almost mask the presence of 

 other grains. 

 Zircon, always present, and sometimes the most abundant species. 

 Magnetite, always present. 

 Tourmaline, always present. 



Hutile, always present, but varies widely in quantity and characters. 

 Staiirolite, often present, but not plentiful. 

 Anatase, occasionally present in noticeable amount, as well-formed 



plates of steel-blue colour. 



Barytes, found in a few cases in large amount. It is probably 



a cement, though well-formed crystal plates occur. Sandstones 



containing this cement are very hard. I find it in several 



building stones. 



Characteristic of all the grains is their angularity. The garnets 



have the dodecahedral cleavage developed out in a remarkable manner, 



^ xibstract of paper read before the British Association, Section C (Geology), 

 Dundee, September, 1912. 



