SECTIONAL TRANSACTIONS.— C. 557 



outcrops of the West of England, but especially in the western half of the Hensbarrow 

 granite. 



China clay rock is an altered granite in which the felspar has been more or less 

 completely changed to kaolin or to amorphous hydrated aluminium silicate. It 

 occurs along the strike of fissure lodes, and has been formed by the action of solutions 

 of magmatic origin. The evidence for this statement may be summarised as follows : — 



(1) The china clay rock is more completely kaolinised near the fissure lodes, 

 ' stent ' lodes, and the completeness of kaolinisation increases with depth. 



(2) The occurrence of the china clay is associated with the occurrence of such 

 minerals as tourmaline, gilbertite and sericitic mica. 



(3) The physical characters of the West of England china clay are markedly 

 different from those of clay of obvious meteoric origin. 



(4) Clay deposits occur with a cover of unaltered granite. 



(5) Though clay deposits have been worked to a depth of 350 ft., no instance is 

 known of a deposit dying out in depth. 



The china clay is washed from the china clay rock by means of monitors, the sand 

 settled in a pit and the clay water pumped to surface and run through a series of long 

 narrow channels, where the fine mica is settled together with any coarse clay and 

 fine quartz. The clay is then settled in large tanks and afterwards transferred to 

 larger tanks about 60 ft. x 40 ft. x 8 ft., which are usually situated along the mineral 

 railway. All water is returned to the clay pit as a shortage of water would stop 

 or seriously hinder production. 



After settling the clay is transferred to a shallow pan heated by hot air flues below. 



China clay is of various grades : paper or bleaching clay, which sells at 45.s. to 655. 

 <a ton ; potting clay, which sells at 25s. to 45s. a ton ; and the product of the first 

 settlement, which is called ' mica,' and sells for 17s. to 22s. a ton if of good quality. 



The cost of producing the clay is very variable, but is never less than 20s. — ^more 

 often 25s. to 30s. — so that the quality of the clay produced is an important factor. 

 The better clays are, moreover, easier to market than the lower qualities. 



The percentage of clay produced from the china clay rock is sometimes as low as 

 10 per cent., while 35 per cent, is a maximum. 



The clay industry is controlled by two associations, one dealing with best quality 

 clay only, the other with inferior clay. These associations limit the output of each 

 company in accordance with the market, but guarantee to each company the sale of 

 a quantity of clay proportionate to the average sales of the year. 



Mr. G. ViBART Douglas. — Some Geological Relationships of the Pyritic 

 and Cupreous Ore-bodies of Htielva, Spain. 



The ore-bodies may be grouped as : (a) contact bodies between porphyry and 

 slate, with the ore either on the flank or as a roof pendant of the porphyry ; (b) massive 

 sulphides entirely in slate ; (c) stockwork and disseminated sulpirides entirely in 

 porphyry ; (d) veins carrying sulphides (of minor importance). 



The present land surface, being a chance surface by erosion, the ore-bodies of the 

 province, as grouped in this classification, represent various horizons rather than 

 isolated types. Thus the idealized section would show a stockwork porphyry passing 

 into massive sulphides on the contact of the porphyry and the country rock (slate, 

 grits, &c.). Above this zone, and entirely in slates, there would be lenticular bodies 

 of sulphides. The outermost manifestations of mineralization are vcinlets of quartz 

 which may carry sulphides. There are numerous examples which illustrate these types. 



With regard to the genesis of the ore-bodies the evidence favours hydrothermal 

 replacement. 



The general sequence of events is as follows : — 



(1) Intrusion of porphyry as dyke and sill into a belt of Lower Carboniferous 

 slates. The cleavage of the slate has a very regular strike and dip, and this has been 

 the controlling factor in the geographical distribution. 



(2) Auto-shearinc and brecciation of the porphyry during injection. 



(3) Alteration of the slates by intrusion of the porphyry, especially where the 

 contact of the porphyry undercuts the slaty cleavage. 



(4) Major fracturing of the porphyry, the result of movements of the magma 

 producing torsional effects in the non-sheared crystalUzed porphyry. 



(5) Injection of hydrothermal solutions along the major fracturing and shear zones. 

 These solutions are thought to be derived from the parent magma, to which the 

 porphyry belongs. 



