712 DYNAMICAL GEOLOGY. 



from any movement of the rocks, however slight or from whatever 

 cause ; they abound in all disturbed and metamorphic beds. They 

 may have great depth, extending through a series of formations, or 

 be confined to particular strata. Where a disturbance is in pro- 

 gress, the different kinds of rock will necessarily be fractured dif- 

 ferently, according to their nature. Those that are unyielding or 

 fragile may be broken into numberless fragments, and these frag- 

 ments widely displaced : so that, when the opened spaces or fissures 

 are filled, the rock will be reticulated with irregular and seemingly 

 faulted veins. The forming of veins by the opening of layers, alluded 

 to above, occurs especially in slate-rocks ; auriferous quartz veins 

 are to a great extent of this character. The general forms and 

 other characteristics of veins are described and illustrated on pages 

 119-123. 



2. Methods of filling veins. — There are three ways of filling veins : (1) 

 by injection from below; (2) by infiltration from above; (3) by 

 infiltration from the enclosing rocks either side of the vein or 

 bounding it along some portion of its course. Under the second 

 and third methods, heat is not absolutely necessary, though generally 

 required. 



First method. — The first method — that by which trap dikes were 

 formed — is not the common one. There are cases, like that of the 

 Lake Superior region (p. 195), where metals or metallic ores are 

 directly associated with injected dikes. But it is always a question, 

 in such a case, whether the metallic ingredient was derived from 

 the same deep igneous source with the melted rock of the dike, or 

 whether it was received from the rocks of the deeper walls of the 

 fissure during the progress of its injection. The vapors or mineral 

 solutions produced at such a time often penetrate the rock ad- 

 joining the veins, sometimes to considerable distances, either dif- 

 fusing ores through them, or filling cracks or long fissures. 



Second method. — The second method is exemplified only in super- 

 ficial veins, seams, or cavities. Carbonate of lime is often thus de- 

 posited in seams or open cavities. 



Third method. — The third method is that by which the great ma- ' 

 jority of the veins in metamorphic rocks, whether simply stony 

 or metalliferous, were produced. The nature of the minerals con- 

 stituting veins, their associations, and the banded structure often 

 characterizing them, are opposed to their formation by injection. 

 An example of the banded structure is represented in fig. 976, in 

 which 1, 3, and 6 are sections of layers of quartz ; 2, 4, of gneissoid 

 granite ; and 5, of gneiss ; and other examples are described on 

 page 123. Such an arrangement could have resulted only from a 



