658 C. LE NEVE FOSTEE ON SOME 



conversion of granite into Greisen along the sides of the small 

 tin-lodes, it is easy to suppose, where the altering action was either 

 very energetic or long continued or the lodes very numerous, that 

 the whole of the intervening rock might be converted into a large 

 mass of Greisen. 



Discussion. 



Mr. Waeington "W. Smyth said that as many of the principal 

 stockworks have been already described, he would pass over the paper 

 dealing with them and proceed to those treating of the origin of 

 other stanniferous deposits, for which some originality seemed fairly to 

 be claimed by the author. He remarked that it had long been known 

 that, although mineral veins often occur between definite walls, some- 

 times one or both of their walls had been attacked by the same 

 forces concerned in the aggregation of the minerals. It seemed 

 likely that the same forces which brought about the deposition of 

 the ores may have also simultaneously altered the neighbouring 

 rocks, and led to the appearance in them of singular minerals, such 

 as those mentioned by the author, and notably substances con- 

 taining fluorine and boracic acid. Miners have observed that oc- 

 casionally the capel or cab is more valuable than the leader or lode 

 itself. In some undoubted instances the capel may have advanced 

 from the fissure into the rock. In killas bordering on granite much 

 schorl often occurs. 



Mr. Deew inquired whether the more irregular masses of minerals 

 do not occur not parallel to the lode, but as if in the lesser branches 

 of fissures. 



Prof. Bonnet found a confirmation of Dr. Le Neve Poster's obser- 

 vations in the structure of luxulianite, the remarkable rock of which 

 the Duke of Wellington's sarcophagus in St. Paul's Cathedral is 

 made. Some schorl rocks are altered granite, others altered killas. 

 He noticed the singular molecular changes in the quartz, schorl, 

 and orthoclase of luxulianite. Wo doubt acidulated waters, charged 

 with the necessary minerals, altered the granite and formed the 

 veins. 



Prof. Seeley remarked that 25 years ago he had been taught by 

 Mr. Warington Smyth and Prof. Eamsay that the rocks near veins 

 of minerals are often altered. Near fissures, no doubt, the rocks 

 had been subjected to great strains, which lessened the cohesion, and 

 thus favoured the infiltration of waters conveying mineral matters 

 in solution into the rocks beyond the actual fissure. 



Mr. Sollas remarked that he had long been puzzled to know what 

 the ordinary theory of mineral veins might be. The facts brought 

 forward this evening supported an idea which some time ago had 

 occurred to himself. The surface of volcanic regions often bears 

 volcanic cones and also mineral springs, the materials of both being 

 brought up through fissures. When denudation removes the sur- 

 rounding country, masses of gabbro and granite bosses are laid bare, 

 and mineral veins appear at the surface. Mineral veins seem to 



