STRUCTURES COMMON TO ALL ROCKS. 233 



the field geologist but also to the practical miner, to know 

 which side has gone up or down ; for valuable beds of 

 coal or veins of metal are thus displaced, and it is impor- 

 tant to know which way they went. Now, normal faults 

 are far the more common, and therefore a very general 

 though not universal rule in such cases is this : In case 

 of inclined fissures, the foot-wall or lower side has gone 

 up, or the hanging wall or upper side has dropped down. 

 Or, it may be otherwise expressed, thus: "The dip or 

 hade (slope) of the fissure is toward the down-throw/' 

 In Fig. 138, which represents an actual section, the rule 

 is followed in every fissure. The exceptions to this rule 

 (Fig. 136) are found only in the strongly folded rocks of 

 mountain-regions. 



SECTION II. MINERAL VEINS. 



Let any one examine rocks, especially metamorphic 

 rocks, in mountain-regions, and he will see that they are 

 marked with seams and scars running in all directions, as 

 if they had been crushed and broken and again mended ; 

 as indeed they were. Now, all such markings and seam- 

 ings, whatever be their nature and origin, are often called 

 by the general name of veins. Thus, beds of coal, or 

 gypsum, or salt, on the one hand, and the fillings of 

 fissures by fused matter, on the other, are sometimes 

 called veins. It is evident that no scientific progress can 

 be made so long as things so different are confounded 

 under the same name. 



Definition. Putting aside, then, all beds formed as 

 sediments at the bottom of water, such as coal, gypsum, 

 etc., and all fillings of fissures by fused matter, such as 

 dikes, etc., veins may be defined as (usually) the fillings 

 of fissures or cnu-ks by slow deposits from solution in per- 

 colating waters, of materials leached from the surround- 

 ing or underlying rocks. Since the deposit takes place 



