ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 125 



obliterated or only shewing itself by the difference of colour 

 known as the "stripe" or "riband." Although usually found 

 among the Palaeozoic rocks, some pretty good slates used for 

 roofing in the Alps are of Lower Eocene age. 



The effect of pressure in producing a cleavage structure in 

 fine-grained sediments, or in similar substances of even com- 

 position, may be illustrated by experimenting with clay mixed 

 with fine scales of mica or of oxide of iron, as shewn by Mr. H. C. 

 Sorby, Professor Tyndall, and others. It was found that the 

 particles all being free to move in direction at right angles to 

 the pressure, those which were scaly in form (unequiaxed 

 particles) became arranged with their broader planes facing the 

 pressure so as to produce a cleavable and laminated mass. 

 When wax alone was used a similar result was obtained, (though 

 it was not so evidently laminated to the sight), as if the particles 

 of wax themselves had been unequiaxial. 



Proceeding from such experiments Mr. Sorby found by the 

 use of the microscope, that the minute particles of which clay- 

 slate was composed were either lengthened in the direction of 

 the cleavage planes, or else re-arranged so that their larger 

 dimensions came to coincide with those planes. 



Mr. Sorby examined many specimens of cleavable limestones 

 from North and South Devon by means of the microscope. He 

 found that the various organisms and patches of crystals had 

 been "compressed and elongated," so as to be "not exactly in 

 the plane either of stratification or cleavage, but in such a 

 direction as is the resultant of their combined influence."* 



All this goes to shew that pressure and cleavage are closely 

 related. Now, as cleavage is usually exhibited in regions which 

 have been much folded and contorted, i.e., exposed to great 

 lateral pressures, it was not unnatural to attribute the cleavage 

 observed to the same cause. But Sedgwick had shewn before 

 1856,+ that cleavage was due to the operation of some very 

 extensive cause after the rocks had undergone great displacements. 

 That this is really the case is proved by the existence of sections 

 like fig. 2, Plate IX, representing the unconformable strata at 



*Phil. Mag. January 1856, page 31. 



f See Phillips's Report on Cleavage, Brit. Assoc. 1857. 



