4i8 SECTIONAL TRANSACTIONS.— C. 



does not differ much from a typical alkalic series, such as the rocks of Oslo 

 district or the British Carboniferous-Permian igneous rocks, except for its 

 considerable larger amount of COj. 



Prof. C. E. TiLLEY, F.R.S. 



Afternoon. 

 Excursion to Wood Ditton and Underwood Hall. 



Tuesday, August 23. 



Dr, S. BucHAN, — Pollution and exhaustion of London's underground water 

 supply (lo.o). 



A general progressive lowering of the level of water in the underground 

 reservoir of London has been taking place over a long period, but during 

 the past few years the faU has become more marked and it is now evident 

 that, unless the fall is checked, parts of the reservoir will be exhausted in 

 35 years or so. The truly artesian conditions of a century ago are gone, 

 and in one area the surface of the water stands 300 ft. lower than it did 

 60 years ago. 



Water is being extracted from the centre of the London Basin more 

 rapidly than it is replenished. Locally, the sands above the Chalk have 

 been drained and dry areas are spreading as the water-level falls, while, in 

 the Chalk, areas are developing from which only a poor supply is likely to 

 be obtained. 



The importance of the geological structures in controlling the distribution 

 of water is becoming apparent now that it is possible to define the principal 

 areas from which the supply is flowing to London. 



Owing to the geological structure the lowering of the water-level has 

 caused brackish water to flow from the tidal reaches of the river Thames 

 into the Chalk and to pollute the supply in an area of high-yielding wells. 

 As the fall in level continues, pollution will become more intense and will 

 affect a greater area of London as well as an increased depth of Chalk. 



The large number of abandoned wells create another potential danger 

 to the water supply. Deterioration of their seal or lining tubes will allow 

 the entry of contaminated water from the superficial deposits to the Chalk. 



Mr. S. I. ToMKEiEFF. — Zonal olivines and their petrogenetic significance 



(10.35)- 



The measurement of the optic axial angle of olivines from various igneous 

 rocks shows that nearly all olivines are zonal. The only exceptions are 

 olivines from ultra-basic rocks and olivines from alkaline acid and inter- 

 mediate rocks. As a rule the zoning is continuous and it shows a pro- 

 gressive enrichment of the mineral in iron towards the periphery. The 

 difference in composition between the centre and the outer rim can reach 

 40 % fayalite, but such cases are rare. In the British Carboniferous 

 dolerites, for example, olivine on the average shows 31 % fayalite in the 

 centre and 39 % in the outer zone, while the olivine from the British 

 Tertiary dolerites shows 18 % fayalite in the centre and 40 % in the 

 outer zone. 



A progressive variation in the average composition of olivine occurring 

 in the different phases of a single intrusive mass is demonstrated by the 



