190 THE FOKMS OF WATER IN 



sharply ; but you can see, by tearing, that it is lami- 

 nated. Let us chill it with ice. We find afterwards 

 that no slate rock ever exhibited so fine a cleavage. 

 The laminae, it need hardly be said, are perpendicular 

 to the pressure. 



486. One cause of this lamination is that the wax is 

 an aggregate of granules the surfaces of which are places 

 of weak cohesion ; and that by the pressure these, 

 granules are squeezed flat, thus producing planes of 

 weakness at right angles to the pressure. 



487. But the main cause of the cleavage 1 take to be 

 the lateral sliding of the particles of wax over each 

 other. Old attachments are thereby severed, which 

 the new ones fail to make good. Thus the tangential 

 sliding produces lamination, as the rails near a station 

 are caused to exfoliate by the gliding of the wheel. 



488. Instead of wax we may take the slate itself, 

 grind it to fine powder, add water, and thus reproduce 

 the pristine mud. By the proper compression of such 

 mud, in one direction, the cleavage is restored. 



489. Call now to mind the evidences we have had 

 of the power of thawing ice to yield to pressure. 

 Recollect the shortening of the Glacier du Geant, and 

 the squeezing of the Glacier de Lechaud, at Trelaporte. 

 Such a substance, slowly acted upon by pressure, will 

 yield laterally. Its particles will slide over each other, 

 the severed attachments being immediately made good 

 by regelation. It will not yield uniformly, but along 



