54 QUIGKSILVEH DEPOSITS OF THE PACIFIC SLOPE. 



absent, niul sonietiines tlie walls of vein-like deposits are not well defined. 

 But veins of ideal structure, such as those upon which the diagrams of text- 

 books are founded, are not common in. all regions, even in gold, silver, or 

 copper deposits. Small veins in hard, coherent rock often assume this sim- 

 ple form, l)ut large veins in volcanic or partially metamorphosed rocks are 

 often indistinctly bounded and are very comple.x; in structure. In many 

 parts of the Comstock lode, for example, there is no definite hanging wall, 

 and the bonanzas of that great vein are masses of brecciated rock filled in 

 with ore. So, too, the gold veins of California are in great part bed veins, 

 a fact due to the nearly vertical position of the strata before the deposition 

 of ore, and they are often somewhat indistinctly defined. In short, the char- 

 acter of the fissure which a vein fills must depend on the physical properties 

 of the rock, and clean-cut open fissures can be formed only in appropriate 

 material. Inmanv cases a fracture will produce a belt of crushed country 

 rock, instead of an open crack, and the ore deposited in the interstitial space 

 will depart to a corresponding degree from an ideal vein. Where the strata 

 of a region have a nearly vertical position prior to the formation of veins, 

 bed veins must prevail. When ore is deposited in contact with porous rocks, 

 such as many sandstones, impregnation must take place. The chief differ- 

 ence between an impregnation in sandstone and the injection of a breccia is 

 that in the former case the interstitial space is due to the original structure 

 of the rock, instead of being brought about by dynamic action accompanv- 

 ing the formation of the main fissure. Impregnations of other ores, as well 

 as those of mercury, are not uncommon. 



Mr. Lipoid showed conclusively that the deposit of Idria consists of 

 simple veins, reticulated masses, and impregnations. Evidence is given 

 above which tends to show that the deposit of Almaden is similar, except 

 that the reticulated masses are tabular and vein-like and that bed veins 

 greatly predominate over those which cut the beds. Humboldt's descrip 

 tion of Huancavelica shows that similar conditions there prevail. At Val- 

 lalta, also, stringers of ore pierce the shales, the porphyry is impregnated, 

 and the main mass of the ore seems to be a somewhat tabular or vein-like 

 stockwork. In short, all the better-known deposits are referable to the 

 thi-ee forms of dei^osits described by Mr. Lipoid, and I know of no sufficient 



