Ch. XXXVI.] SLATE ROCK OF NORTH DEVON. 751 



movement, although, they are much fewer thau in the slaty strata 

 above and below. 



Above the sandy bed df, the stratum cis somewhat disturbed, while 

 the next bed b is much less so, and a not at. all ; yet all these beds c, 6, 

 and a, must have undergone an equal amount of pressure with d, the 

 points a and# having approximated as much towards each other as have 

 d and/. The same phenomena are also repeated in the beds below d, 

 and might have been shown, had the section been extended downwards. 

 Hence it appears that the finer beds have been squeezed into a fourth of 

 the space they previously occupied, partly by condensation, or the closer 

 packing of their ultimate particles (which has given rise to the great 

 specific gravity of such slates), and partly by elongation in the line of 

 the clip of the cleavage, of which the general direction is perpendicular 

 to that of the pressure. " These and numerous other cases in North 

 Devon are analogous," says Mr. Sorby, "to what would occur if a 

 strip of paper were included in a mass of some soft plastic material 

 which would readily change its dimensions. If the whole were then 

 compressed in the direction of the length of the strip of paper, it 

 would be bent and puckered up into contortions, whilst the plastic 

 material would readily change its dimensions without undergoing such 

 contortions ; and the difference in distance of the ends of the paper, 

 as measured in a direct line or along it, would indicate the change in 

 the dimensions of the plastic material." 



The studeut will readily conceive that, when the shape of a fossil 

 or of a crystal of some mineral, or of a spheroidal concretion, has 

 been altered by lateral pressure, the new forms which they assume 

 respectively will vary according to whether they have yielded in one 

 or more directions. They may have been drawn out solely in the 

 direction of the dip of the cleavage, or they may have yielded in a 

 plane perpendicular to that dip, or they may have undergone both 

 these movements. By microscopic examination of minute crystals, 

 and by other observations too minute to be detailed here, Mr. Sorby 

 comes to the conclusion that the absolute condensation of the slate 

 rocks amounts upon an average to about one-half their original vol- 

 ume. This must have resulted chiefly from the forcing of the particles 

 more closely together, so as to fill up the spaces left between them, 

 when they only touched each other. The rest of the change has been 

 due to elongation which has produced slaty cleavage. 



Most of the scales of mica occurring in certain slates examined by 

 Mr. Sorby lie in the plane of cleavage ; whereas in a similar rock not 

 exhibiting cleavage they lie with their longer axes in all directions. 

 May not their position in the slates have been determined by the 

 movement of elongation before alluded to ? To illustrate this theory 

 some scales of oxide of iron were mixed with soft pipe-clay in such a 

 manner that they inclined in all directions. The dimensions of the 

 mass were then changed artificially to a similar extent to what has 

 occurred in slate rocks, and the pipe-clay was then dried and baked. 



