﻿300 ME. A. M. ITNLAYSON ON ORE-DEPOSITION [May 1 9 JO, 



II. SOUKCE OE THE ORES. 



Since the days when the lateral- secretion theory was first pro- 

 pounded by Sandberger, deposits of lead and zinc ores have afforded 

 much scope for investigation of the source of the metals. In Great 

 Britain there are two general types : namely, veins in the Carboni- 

 ferous Limestone, and veins in the older slates and granites. Con- 

 cerning the latter type, there is no clear ground for argument ; the 

 ores have obviously been derived from a deep source, probably from 

 deep-seated igneous masses. Attention is, therefore, here confined 

 to the source of the ores in the limestone districts. 



Fluorine. — Four samples of limestone were analysed by the 

 method of AY. F. Hillebrand, 1 with the following results : — 



Locality. Fluorine percentage. 



Halkyn (Flintshire) 021 



Matlock (Derbyshi re) , Oil 



Nenthead (Cumberland) 004 



Bookhope (Durham) 0*13 



Duplicate analyses of these specimens yielded sometimes discordant 

 results, due probably to imperfections in the method of assay, as 

 pointed out by Hillebrand ; but the results leave no doubt as to 

 the presence of small quantities of fluorine in the limestones. The 

 constituent has probably been derived from seaweeds or other 

 organic matter in the original sediments, or from the fluorine- 

 bearing calcium phosphate of shells. 



It is highly improbable, however, that this can be the source of 

 the fluorine in the vein-fluorspar. The quantities present are 

 extremely small, and the occurrence of fluorine in the rocks bears 

 no relation to the occurrence of fluorspar in the veins. The occur- 

 rence and mineralogical relations of the fluorspar indicate that 

 fluorine has been one of the original elements in the vein-solutions. 

 The presence of fluorine is believed to indicate, in short, a deep- 

 seated origin, dating probably from processes of magmatic extraction. 



Lead and Zinc. — In searching for these constituents, a series of 

 rocks was examined, the material being ground very fine. Quanti- 

 ties of from 30 to 100 grams were taken, and analysed by methods 

 previously used by W. F. Hillebrand 2 and by James D. Eobertson. 3 

 The following table gives the results : — 



1 'Analysis of Silicate & Carbonate Bocks' Eull. U.S. Geol. Surv. No. 305 

 (1907) p. 157. . 



2 ' Geology & Mining Industry of Leadville, Colorado ' Monogr. xii, U.S. 

 Geol. Surv. 1886, p. 591. 



3 GeoL Surv. Missouri, vol. vii (1894) pp. 479, 740. 



