﻿Vol. 66.~\ IN THE LEAD AND ZINC VEINS OF GKEAT BRITAIN. 305 



and granular, and is transformed to an aggregate of siderite, sericite, 

 and quartz. Ilmenite alters first to leucoxene, and this, together 

 with magnetite, changes largely to siderite, which occurs in abun- 

 dant greyish-brown patches. The rest of the rock is composed of 

 granular secondary quartz and much finely divided sericite, while 

 fresh apatite needles are scattered through it. As seen from the 

 analyses, the principal chemical changes have been a loss of titanium 

 and ferric oxides and a gain in silica, potash, and carbon dioxide. 

 Taking the major constituents, and recalculating the analyses, we 

 obtain the following approximate mineralogical compositions : — 



C 1 (fresh). C 2 (altered). 



Augite 34-86 Siderite 14/42 



Magnetite and ilmenite . 9 - 92 Sericite 32-58 



Plagioclase(andesine)... 5269 Quartz 4929 



Apatite 074 Apatite 074 



Totals 98-21 97'03 



The intense changes in this rock have been essentially mineralo- 

 gical, there being no marked percentage loss of bases in the altered 

 rock. 



The Foxdale granite also shows very considerable alteration. 

 The fresh rock is a binary granite, containing orthoclase and oligo- 

 clase, with some microcline, muscovite, and frequently biotite, and 

 abundant quartz. Adjoining the Foxdale vein the rock is pulveru- 

 lent and earthy, and of a greenish-yellow colour. It crumbles 

 readily, and swells on the addition of water. Under the microscope 

 the quartz and muscovite are seen to be unchanged, but the felspars 

 are completely altered to aggregates of sericite. Patches of calcite 

 are frequent, and fine mosaics of secondary quartz sometimes replace 

 felspar phenocrysts. Biotite is first bleached, and later altered to 

 pale-green chlorite with separation of magnetite. The analyses 

 (D 1 & D 2 ) show a loss of silica, iron oxides, magnesia, and soda, 

 and a concentration of lime, potash, and carbon dioxide. The 

 changes are in general comparable with those in the other rocks just 

 described, and are in many respects similar to the alteration of the 

 gneiss which carries the silver-lead veins of Freiberg, except for the 

 great loss of constituents in the altered rock at Freiberg. 1 



The specific gravities tabulated on p. 304 show that the altered 

 rocks have in general a reduced specific gravity. This, without 

 marked loss of constituents, is in accordance with the alteration to 

 a less dense mineral aggregate. The marked increase in specific 

 gravity of the altered granite of Foxdale (D 2 ) indicates greater 

 porosity, a property which is revealed by its tendency to swell and 

 disintegrate when exposed in the mine-workings. The accom- 

 panying diagrams (fig. 1, p. 306), constructed on W. H. Hobbs's 



1 Th. Scheerer, Zeitschr. Deutsch. Geol. Gesellsch. vol. xiv (1862) p. 87; 

 and A. Stelzner, Neues Jahrb. toI. i (1884) p. 271. 



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