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MR. A. M. FINLAYSON ON ORE-DEPOSITION [May I9IO, 



No. 



1 

 2 



3 



4 

 5 

 6 



Position of Sample. 



Lead. 



Zinc. 



Limestone in the vein. 



Do. on the vein-wall. 

 Do. 10 feet from the vein. 

 Do. 20 „ 

 Do. 40 „ 

 Do. 70 „ 



Per cent. 

 012 



•025 

 •015 



•001 



•002 

 •0005 



Per cent. 

 •040 

 •015 

 •030 

 •004 



•001 

 •001 



There is no uniform variation in passing from the vein into the 

 country-rock, but the analyses clearly show a much higher metal- 

 content in the limestone adjoining the vein than in the rock farther 

 away. A similar phenomenon has been recorded by J. S. Curtis at 

 Eureka (Nevada), 1 and the difference is certainly due to impregnation 

 of the adjacent rock by the vein-solutions. 



(2) A strong objection to the lateral-secretion theory is the close 

 similarity in all respects between veins in limestone, in granite, and 

 in older slates, especially in adjoining districts. Thus the veins in 

 the limestone districts of Alston Moor and Dufton Fell are exactly- 

 similar mineralogically to the veins in the slates and ash-beds of the 

 Lake District. Again, in the Isle of Man there are veins in granite 

 extending into the adjoining slates, numerous veins in the slates 

 themselves, and veins in the Carboniferous Limestone at Castle- 

 town. It is not conceivable that the ores of these different veins 

 can have different sources of origin. 



(3) In the United States, where this problem has been very fully 

 investigated, there are two general types of lead and zinc-ore de- 

 posits : firstly the silver-lead or ' hard-lead ' ores, typified at Leadville, 

 Aspen, Rico, Tin tic, Coeur d'Alene, and Eureka ; and secondly the 

 • soft-lead ' and zinc-ores of the Mississippi and Missouri districts, 

 including "Wisconsin, the Ozark district, South-Eastern Missouri, and 

 Northern Arkansas. The general conclusions in regard to these two 

 types are that the silver-lead ores are of deep-seated origin, while 

 the ores of the Missouri type have been concentrated in most cases 

 from a disseminated condition in the same formation or in one 

 adjoining that in which they are now found. The essential features 

 of the deposits of this type, the sedigenetic deposits of H. E. Bain, 2 

 are their sporadic occurrence, the absence of defined veins, the non- 

 persistent and shallow character of the deposits, and the generally 

 negligible silver-content of the galena. 



Comparing the British occurrences with these conditions, the veins 

 of the limestone areas occur typically in strong fault-fissures, alto- 

 gether distinct from the shallow non-persistent deposits of Wisconsin. 



1 ' Silver-Lead Deposits of Eureka, Nevada ' Monogr. vii, U.S. Geol. Surv. 

 1884, p. 85. 



2 'Economic Geology vol. i (1906) p. 331. 



