N'otes on the Area of Intrusive B.ocks at Davgo. 151 



the walls of the lode, but it is very commonly the case that 

 the rocks at each side are also not only much altered 

 mineralogically, but also more or less impregnated with ores. 

 The mineral alterations in the walls of the lode are more 

 recognisable where the lode passes down into the plutonic 

 rocks than where it is in contact schists. As an example of 

 such alterations, I may quote the Eureka mine at Swift's 

 Creek, where the reef passes through massive quartz mica 

 diorites, I observed that the foot- wall of the lode was much 

 lighter in colour than the hanging wall, as well as being 

 impregnated with ordinary pyrites. A hand sample of the 

 rock is crystalline-granul?irj and greenish yellow in tint. In 

 a thin slice it can be seen that the felspars have been so 

 much altered that no traces of any twinning remain, the 

 crystals being either entirely kaolinised, or where less altered, 

 having the appearance of one of the pinite minerals. Tiaces 

 of the former presence of iron-magnesia mica remain as 

 fibres of chlorite, and the quartz is of two kinds — namely, 

 the crystalline grains of the original rock and a second 

 generation of much smaller and very interlocking granules. 



In some cases I have observed, in addition to such mineral 

 alterations, that minerals in the walls of the lode have been 

 structurally altered b}^ crushing. 



The extensive impregnation of this class of quartz lodes 

 with various kinds of ores, the banded structure of some of 

 the quartz veins, and the frequent restriction of the gold to 

 some of the bands rather than to the others, the imj^reg- 

 nation of the walls of the fissures with ores, and the exten- 

 sive mineral alterations which have been made in the 

 bounding rocks, all point, when taken together, to the 

 formation of this class of contact veins by the action of 

 aqueous solutions charged with mineral and metallic 

 materials. It is probable that these solutions were heated^ 

 although not necessarily to any high temperature, for the 

 observations made by Daubree on the effects produced by 

 the thermal waters at Plombieres on the Roman masonry, 

 and metallic objects therewith, show that a comparatively 

 low temperature will suffice, even at the earth's surface, to 

 bring about mineral alteration and the form.ation of ores.* 



* Memoire sur la relation des sources tliermales de Plombieres, &c. 

 Annales des Mines, 1858, XIII., p. 227. Etudes et experiences synthetiques 

 sur la Metamorphism, Memoires presentes a I'Academie des Sciences, XVII., 

 p. 98. 



