1 18 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



"master" joints, "backs" and "beds," which continue uninter- 

 ruptedly for considerable distances, while the second includes 

 such " joints," " heads," or " cutters " as are much more limited 

 in their range. The master joints pass indiscriminately through 

 the whole series, while the beds* separate the various strata. 

 The secondary joints, on the contrary, are confined to each 

 separate stratum. Sometimes, indeed, they do not pass quite 

 through a block, in which case they are known to quarrymen as 

 " shakes." 



Both master joints and secondary joints are sometimes 

 curved, though usually approximately plane. 



Near the surface, in a cliff or quarry, the joints are usually 

 more or less open, and often discoloured by chemical and other 

 changes, so that they become prominent features. At greater 

 depths the joints are much closer and altogether less conspicuous, 

 yet they are really present and may soon be discovered if the 

 rocks be brought to the surface and exposed to atmospheric 

 influences for a short time. 



The main joints, known as " beds " in aqueous or metamor- 

 phic rocks, are directly traceable to the want of continuity arising 

 from differences of composition, or from interruptions in the 

 process of deposition; while the so-called beds in granite and 

 other eruptive rocks are more probably due to zonal cooling and 

 consolidation. Other main joints in all classes of rocks seem to 

 have their directions determined by the directions of the axes 

 of elevation, these elevations being due, as already suggested, 

 to the existence of pressures transverse to their directions. But, 

 when the pressure is removed by a local subsidence and replaced 

 by a strain, or when it is relaxed owing to shrinkage of the 

 mass, we may expect a series of cracks to be formed. A little 

 consideration will shew that these must usually run parallel to 

 the elevations and approximately at right angles to the beds. 



But contraction is all this while going on throughout the 

 mass, and when the time has come for the formation of another 



*The term " bed" has three common meanings. Thus it is (1) the stratum 

 or layer of rock itself, (2) the main joint between two such, and (3) that surface 

 of a block which is parallel to the stratification. It is generally easy to determine 

 from the context in which sense the word is used. 



