326 SECTIONAL TRANSACTIONS.— C. 



contrast, though this change probably set in with varying vigour in different areas : 

 locally basal gravels suggest transitional conditions of moisture, and it is to this 

 horizon that the Ordos Palaeolithic culture belongs. 



During Pliocene times chemical weathering had penetrated deep into the surface 

 of the Mongolian plain, but the residual decay products remained undisturbed. 



In Pleistocene times ujflift exposed this material to high winds that swept it over 

 the border into China, depositing it as a blanket of loess as far south as the Yangtze 

 valley. A characteristic fossil is the egg of the ostrich, Struthiolithus. 



During the fluctuating stage of return to moister conditions, wind and water vied 

 as agents of scour and deposit. The 'Dune-dwellers of Shabarakh ' (?Azilian), 

 whose implements were found by the Central Asiatic Expedition, presumably fall 

 within this stage. The various Chinese Neolithic culture stages invariably lie on the 

 upper surface of the main body of the torrential deposits of this Panch'iao stage. 



The present stage of erosion has been stimulated by slight up- warping of the moun- 

 tain hinterland. The coast-line of the Yellow Sea shows evidence of emergence only 

 in one locality : elsewhere all points to a continued geosynclinal sinking of the 

 delta area. 



Mr. E. H. Davison. — The Variation in the Composition of Cornish Granites 

 and its Relation to the Occurrence of Tin Lodes. 



It has long been recognised that the Cornish tin lodes which occur in the granite 

 are often accompanied by hydrothermal and other alterations of the country rock 

 such as greisening, tourmalinisation, kaolinisation, &c. These intense types of 

 alteration are usually confined to the immediate neighbourhood of the lode fissure, 

 though in some cases they spread outwards for a considerable distance and cover 

 large areas. The presence of any of these types of alteration is invariably 

 suggestive of the presence of mineral lodes. 



In many of the mining districts, however, the granite is for the greater part not 

 affected by these drastic changes, and the aim of the work described in this paper was 

 to discover if there is any constant difference in composition between the granite in 

 areas free from lodes and that of the granite in areas where lodes are numerous. If 

 any such difference in composition exists it would be of great help in deciding which 

 areas are worth prospecting and which are not. 



In order to solve the problem samples of granite were collected from as many 

 localities as possible, including surface exposures in quarries and cliffs as well as under- 

 ground exposures in mines. From these samples thin sections were cut and examined 

 microscopically, while in many cases the rock was subjected to mechanical analysis to 

 determine the proportions of significant minerals. As a result the granite was found 

 to vary in composition as follows : — 



Granite from areas free from mineral lodes. 



Muscovite seldom present. 



Biotite usually present in an unaltered condition. 



Little or no tourmaline. 



Fresh felspars free from kaolinisation or sericitisation ; well-formed crystals of 

 felspar. 



Granite from areas with mineral lodes. 

 Muscovite always present. 



Little or no biotite. When present the biotite shows alteration to chlorite. 

 Tourmaline almost always present. 

 Felspars show partial sericitisation and occur in ill-formed crystals. 



A further problem was afterwards attacked. It is a common experience to find 

 that lodes vary in value along their strike — that is, they have zones of rich ore alter- 

 nating with zones of low-grade ore which usually run diagonally down the dip of 

 the lode. An attempt was therefore made to discover if there was an easily 

 recognisable difference in composition between the granite near the poor (low-grade) 

 lode and that near the rich (high-grade) lode. 



The aid of the various mine managers was asked for and they kindly arranged for 

 the collection of samples of the granite from near high-grade lodes and also from 

 near low-grade lodes. These samples were treated as described above and the 

 granite was found to vary as follows : — ■ 



