344 J. Arthur rniLLirs on the rocks of 



elvanite, proposed by Jukes, being now accepted by many petro- 

 logists as a synonym for quartz-porphyry, he was anxious to know 

 whether it was in this sense that Mr. Phillips employed the term. 

 He felt that the author's statements concerning the microscopic 

 characters of killas were very just, since he had occasionally met 

 with instances in which it was not easy to discriminate micro- 

 scopically between slaty and felsitic matter ; and he commented on 

 the difficulty often experienced in determining some of the green 

 hydrous silicates which occur as alteration-products in eruptive rocks, 

 suggesting that in the present state of microscopic inquiry it might 

 be well to adopt the somewhat vague term " Viridite," which was 

 proposed by Vogelsang as a convenient name by which to designate 

 these undetermined chloritic and serpentinous minerals. He could 

 not agree with Prof. Hull in regarding serpentines as altered diorites, 

 as he had not hitherto met with any instances in which the micro- 

 scopic character of serpentine would warrant such a conclusion. 



Mr. Judd remarked that, in addition to the negative evidence 

 adduced by the author that by no process of metamorphism could 

 the killas of Cornwall be transformed into the material of which the 

 great bosses of granite and the elvans are composed, we have the 

 positive evidence that not only is the composition of these latter, as 

 shown by the author's analyses, identical, but it is also the same as 

 that of lavas which play a very important part in the structure of 

 the globe — namely, the rhyolites or quartz- trachytes. These lavas, 

 moreover, when crystallized, are found to be made up of precisely 

 the same mineral species as granite — namely, orthoclase, quartz, 

 hornblende, or mica, and some triclinic felspar. 



Mr. Collins asked where sulphate of baryta occurred as a common 

 constituent of a lode, and where protogine was to be met with. 



Mr. Forbes did not agree with the author that metallic lodes 

 had been formed by aqueous action, but considered that their 

 metallic contents have in the main come into them from below by 

 sublimation or injection, although afterwards the contents of the 

 veins have been greatly altered by aqueous action, and other minerals 

 thus introduced. 



Mr. Htjlke inquired why the action of sublimation and aqueous 

 conditions may not have been simultaneous. 



Mr. Whitaker suggested that veins may be formed in diiferent 

 ways. He cited the case of blown sand being cemented by copper, 

 when water from a copper mine percolated through it, and of the 

 cupriferous beds in the New Ked at Alderley Edge ; and he thought 

 that if water thus dissolves copper, then some veins may have been 

 formed by infiltration from the surface. 



Mr. Koch said that he had been able to compare Saxony with 

 Cornwall, and was much struck by the similarity of the rocks and 

 minerals in the tin-districts of both countries. In fact, after visit- 

 ing Saxony, he had been able to recognize Greisen and Zwitter- 

 rock in Cornwall associated as in Saxony. He thought steam, 

 charged with fluoric acid, had played a great part in the formation 

 and alteration of dykes and veins. He had found that granite 



