Ch. 29] VEIN DEPOSITS IN SEDIMENTARY ROCKS 527 



Many veins, particularly the stronger quartz and pegmatite varie- 

 ties, cut indiscriminately across igneous-sedimentary contacts. In the 

 sedimentary rocks they eventually die out, gradually changing char- 

 acter according to general rules, such as from coarse-veined quartz to 

 fine-grained chalcedony, accompanied by irregular silicification effect 

 on the wall rocks. These changes, however, are not likely to be well 

 exhibited within the limits of a particular vein system or locality; 

 they are apparent only from a study of broad regional patterns. 



Other veins, such as those of barite and fluorite, are found much 

 more often within sedimentary rocks than in igneous rocks, and often 

 far from any known intrusives of possible relationship. This is true 

 also of many carbonate veins. 



With respect to all these diverse types of veins, the chief importance 

 of the sedimentary rock as a host lies in its bedded character and the 

 resultant alternation of competent and incompetent types. As a rule 

 the veins form freely only in competent rocks, which were strong 

 enough, under the load they happened to carry at the depth of vein 

 formation, to maintain openings of visible size and good continuity. 

 They may be essentially free of rubble and breccia fragments or filled 

 with these in nearly any degree so long as good continuity for the 

 flow of mineralizing fluids is achieved. It follows, therefore, that hard 

 or brittle rocks, such as quartzite, or sediments hardened to a con- 

 siderable degree by metamorphism, are much better vein hosts than 

 soft shales and unconsolidated sediment. Competent rocks with low 

 porosity, also, are better hosts to veins than porous sediments which 

 tend to divert the fluids from vein fractures into lateral bedding planes. 

 Unmetamorphosed limestones are of intermediate character, reason- 

 ably favorable receptacles in some cases, particularly when they are 

 massive and thick-bedded; poor in many places, especially if thin- 

 bedded and shaly in nature. 



As bedded formations are always of limited thickness and likely to 

 alternate in thin successions, it follows that vein formation is favored 

 when the attitude of the beds somewhat accommodates the tendency of 

 vein material to rise, and is inhibited when beds lie flat or otherwise 

 athwart the general course of any given vein. A series of flat-lying 

 rocks traversed by vertical veins makes for interruptions in continu- 

 ity and values. Productive portions are likely to be restricted to 

 limited vertical stretches within competent horizons. Under these con- 

 ditions the value of a particular favorable horizon is likely to be pro- 

 portionate to its thickness. A steeply dipping series of sedimentary 

 beds with some brittle and competent members favors continuity of 



