130 OVERLYING AND TRAP ROCKS. 



The more obvious veins vary in size from a breadth 

 of many yards to the thickness of a thread ; these 

 minute ones occurring, as far as I have seen, only 

 among the primary strata. They do not often ra- 

 mify ; but that occurrence is most frequent in those 

 of greenstone, claystone, and basalt. They are gene- 

 rally straight, and often remarkably so, though occa- 

 sionally bent, and even curved at right angles. The 

 extent which they traverse without undergoing any 

 variation of size, is sometimes very considerable ; 

 reaching even to many miles ; as is the case near 

 Comrie in Scotland ; while, in other instances, they 

 vanish soon after they quit the parent mass. As 

 many of them originate in visible masses, and termi- 

 nate in the surface after a certain course, we might 

 infer that their depth downward was also limited, even 

 were there not abundant evidence of this, everywhere. 

 To assert that because, in some cases, their depth has 

 not been reached by miners, they must be indefinite 

 downwards, as was done by writers still quoted for 

 authorities, is to be ignorant of the nature of veins ; 

 as it is to forget, that while a horizontal extent of a 

 few hundred yards is a very short course, the same 

 dimensions downwards, equally limited, would carry 

 them out of the reach of mining operations. For 

 other obvious reasons, this is the supposition of igno- 

 rance ; as their depth must inevitably be regulated by 

 the position of the parent masses. If there are veins 

 which cannot be traced to any source, we may still 

 infer the same superficial immediate origin, if we 

 choose, because we know that such masses often dis- 

 appear under the usual waste. In Sky, for example, 

 where a great number of parallel and similar veins 

 exist together in one spot, only one or two remain 

 connected with the overlying mass, which has evi- 



