32 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



country were William Smith's general and county maps, 

 published between the years 1815 and 1824. In the 

 year 1832 De la Beche made proposals to the Board of 

 Ordnance to colour the ordnance-maps geologically, and 

 a sum of 300/. was granted for the purpose. Out of 

 this small beginning grew the important work of the 

 Geological Survey. 



The cause of slaty cleavage had long been one of 

 the great difficulties of geology. Sedgwick suggested 

 that it was produced by the action of crystalline or 

 polar forces. According to this view miles and miles 

 of country, comprising great mountain masses, were 

 neither more nor less than parts of a gigantic crystal. 

 Sharpe, however, called attention to the fact that shells 

 and other fossils contained in slate rocks are compressed 

 in a direction at right angles to the planes of cleavage, 

 as if the rocks had been seized in the jaws of a gigantic 

 vice. Sorby first maintained that the cleavage itself 

 was due to pressure. He observed slate rocks contain- 

 ing small plates of mica, and that the effect of pressure 

 would tend to arrange these plates with their flat sur- 

 faces perpendicu'ar to the direction of the pressure. 

 Tyndall has since shown that the presence of flat flakes 

 is not necessary. He proved by experiment that pure 

 wax could be made by pressure to split into plates of 

 great tenuity, which he attributes mainly to the lateral 

 sliding of the particles of the wax over each other ; and 

 thus the result of pressure on such a mass is to develop 

 a fissile structure similar to that produced in wax on a 

 small scale, or on a great one in the slate rocks of Cum- 

 berland or Wales. 



The difficult problem of the conditions under which 



