STRUCTURES COMMON TO ALL ROCKS. 237 



vein is here lost by slips ; which way shall we go to recover 

 it ? Eemember the rule already given on page 233 : " The 

 slope or dip or ' hade ' of the displacing fissure (here a) is 

 toward the down-throw/' This rule is not invariable, but 

 very general. 



Surface-Changes. We must not imagine that metal- 

 liferous veins outcrop on the surface in the form we have 

 described. If they contain no metal, veins may indeed 

 appear unchanged on the surface. Quartz-veins may, for 

 example, be often traced over hillsides by strewed frag- 

 ments of white quartz. But metalliferous veins are usually 

 so greatly changed on the surface that, without much ex- 

 perience, we would not recognize them at all. Precisely 

 as rocks are usually concealed by soil resulting from sur- 

 face-decomposition, so veins are concealed by surface- 

 changes. To the experienced eye these surface-changes 

 become surface-5^ws, and are therefore of the greatest 

 practical importance. These surface-signs are far too 

 complex and various to be explained here. We only 

 mention them to guard the pupil against supposing that 

 it is easy to see what we have described above, and to 

 stimulate him to observe for himself. 



Origin of Mineral Veins. This is a difficult and 

 obscure subject, but the following propositions are prob- 

 ably true : 1. Veins have been formed by deposit of min- 

 eral matters from solutions in percolating or subterranean 

 waters. 2. The movement of the subterranean waters may 

 have been in any direction, but mostly up-coming. 3. 

 The waters may have been at any temperature, but mostly 

 hot. 4. The water-ways may have been of any kind, but 

 the openest water-ways the highways of ascending waters 

 are open fissures. 5. The waters have been usually, 

 though perhaps not always, alkaline, i. e., containing 

 alkaline carbonates or alkaline sulphides, or both. 



