124 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



ation is generally parallel to the bedding, and when this is not 

 the case it indicates what is known as "false-bedding." This 

 form of cleavage-structure may, of course, become more evident 

 by subsequent change. Ordinary shale (soft killas) is a good 

 example of a truly laminated rock. 



(2). A laminar arrangement of dissimilar mineral constitu- 

 ents, mostly crystalline and more or less inter-penetrating, is 

 called foliation. It no doubt indicates an original lamination of 

 some kind in most cases, if not invariably, but as it commonly 

 results from subsequent changes, both of structure and com- 

 position, it will be more fully referred to hereafter. Mica-schist 

 is a good example of a foliated rock. 



(3). When a rock consists in any large degree of crystalline 

 components, having marked and distinct cleavages, an arrange- 

 ment of those components in such a way as to make the principal 

 cleavages coincide in direction, must necessarily give rise to a 

 distinct cleavage- structure in the whole mass. Thus, in fig. 1, 

 Plate IX, which roughly represents a block of granite, if the crys- 

 tals of felspar occur irregularly, as shown in the lower part of the 

 figure, the rock will not be cleavable : but if as in the upper 

 part, it will be readily cleavable. This may be called quarry 

 cleavage ; it is common in many rocks beside granite. Such a 

 structure is often original, but there is reason to believe that it 

 may also be induced in some cases. As these cleavages do not 

 affect the soundness of the block, the quarryman working in the 

 granite quarries of the West of England, where such quarry- 

 cleavage is common, is often enabled to cleave out " posts " 12 

 or 14 feet long, whose cross-section is less than 100 inches area. 



(4). Coincidence in direction of the particles of greatly 

 unequal dimensions. This is best seen in roofing-slate, and it is 

 known as true slaty cleavage. This structure is produced by 

 pressure and "shear," and although the structure is usually 

 independent of the three causes already mentioned, yet it is 

 obvious that it may be aided by them. 



How slaty cleavage is produced. Clay slate is essentially an 

 altered mud; it differs from shale in being more highly indurated, 

 in having a more perfect cleavage, and in that cleavage being 

 independent of the bedding, this latter being usually either 



