26 LeConte and Rising— Metalliferous Vein-formation 

 ;.— At first sight the volca 

 oval ash -heap, 300 yardi 



which on analysis is found to be pure silica. It is evidently 

 the residue resulting from complete decomposition of the vol- 

 canic rock, as will be explained hereafter. As we go deeper 

 the rock becomes sounder and the mass now seems to consist 

 of andesite blocks rounded by decomposition with the pro- 

 ducts of decomposition between, and looking much like 

 rounded "bowlders" imbedded in a white tish\ ..ndudky earth. 

 Very often the rounded blocks (so-called bowlders) disintegrate 

 into concentric whitish shells which scale off to a darker 

 sounder rock in the center. These shells are undoubtedly the 

 evidence of an obscure original ball-structure developed by 

 decomposition. In some places an obscure flow-structure is 

 developed in the same way and gives rise to an appearance of 

 Between the loosened shells or laminae, 

 in the earthy mass between the bowlders, and in every crack 

 or crevice of" any kind is found sulphur in abundance and often 

 in very beautiful crystals. As we go still deeper, the rock 

 becomes still sounder until it. finally assumes its natural square- 

 jointed structure. Cinnabar now begins to appear mingled 

 with the sulphur, and in increasi ons, as \ye 



go deeper and deeper. As a general rule therefore, even in 

 portion of the mines, the sulphur is more 

 abundant near the surface and the cinnabar at greater depth. 

 In fact the sulphur is not found at all below a depth of a few 

 feet. The mines were therefore at first worked wholly at the 

 surface for sulphur only, then for sulphur and cinnabar, and 

 now. since regular mining has commenced, for cinnabar only. 



Iron is also very abundant ; near the surface as ferric and 

 magnetic oxides staining in places the disintegrated mass and 

 r greater depth- a- p\rite- n pi.-- tau th< -..m 

 rock. In many places, scattered irregularly through the dis- 

 integrating mass, are found streaks or pockets, or larger areas 

 of a black spongy material consisting of rotten rock colored 

 with magnetic oxide. This material is usually rich in both 

 sulphur and cinnabar. Again: the decomposed material is in 

 many places strongly acid and astringent from the presence of 

 five sulphuric acid and sulphates of alumina and iron. Lastly, 

 bitumen is found every where in very small quantities impreg- 

 nating the rocks, and detectable as minute black globules 

 under the microscope. It is also found in the assay of the 

 rocks. 



As far as yet described the rocks have evidently been 



