﻿322 MR. A. M. FINLAYSON ON ORE-DEPOSITION [May 1910, 



for, while resistant beds may be most favourable in one district, the 

 Assuring may be concentrated in the softer beds in another area. 

 The influence of the country-rock has been due, in these cases, to 

 its physical character, not to its chemical composition. 



In limestone areas the effects are more complex, since they 

 depend on both physical and chemical characters. The influence 

 of the former is well seen in the pinching and dissipation of the 

 fissures in the soft shales of Flintshire and North Derbyshire, and 

 likewise in the hard igneous rocks of Derbyshire and Northumber- 

 land. The most favourable beds for Assuring have been the 

 limestones and cherts, beds intermediate in strength between tho 

 shales and the igneous rocks. The effect of porosity is also well 

 illustrated, as both the impervious shales and igneous rocks have 

 impeded circulation, the ores being concentrated in the intervening 

 horizons of porous calcareous beds. The chemical influence of the 

 rocks has been important, as seen in the concentration of ore in 

 the calcareous and dolomitic beds. These effects are discussed 

 in connexion with the subjoined experimental results. 



Deposition of metals from solution. — In order to obtain 

 information concerning ore-deposition in the presence of different 

 rocks, a series of tests was carried out to compare the quantities 

 of lead and zinc deposited from solutions of their salts by various 

 solid materials. The solutions used were lead nitrate and zinc 

 sulphate of corresponding strength. The solid materials were as 

 follows : — 



1. Washed kaolin. 



2. Calcareous grit, Leadhills. (CaO = 10'5 per cent.) 



o. Cherty limestone, Halkyn. (CaC0 3 =70*45 ; Si0 2 = 20*25 per cent.) 



4. Shell limestone, Alston. (CaC0 3 = 90*50 per cent. ; Organic matter = 



6*60.) 



5. Magnesian limestone. (MgC0 3 =36 per cent.) 



6. Crystalline dolomite. 



7. Crystalline calcite. 



8. ' Curly' shale, Broxburn. (Hydrocarbon =22 '85; Fixed carbon=4'43 ; 



ash =71 '25 per cent.) 



9. Burnt shale, Broxburn. (C = 5*38; SiO., = 5600; Al 2 O 8 =31*20; 



FeX> 3 = 2*84 ; SO 3 = 5-06 per cent.) 



10. Coke, Broxburn. (C=95 per cent.) 



11. Solid paraffin, Broxburn. (Heavy hydrocarbons.) 



The materials were ground to a uniform degree of fineness, and 

 amounts of 10 grams were placed in stoppered jars with 50 cubic 

 centimetres of the solutions. The jars were left for 4 or 5 days at 

 ordinary room-temperatures, with periodical shaking. The solutions, 

 which had been previously standardized, were then filtered off and 

 analysed, to determine the loss of metals in each case. The exact 

 degree of fineness of the powder beyond a certain point makes little 

 difference to the result, provided the solid presents a sufficient 

 surface to the solution to enable it to exert the maximum effect. 

 Again, prolonged contact does not result in corresponding effect, 

 most of the deposition taking place during the first and second days. 

 The percentage results of the tests are recorded in the following 

 table ; — 



