358 MOLECULAR METAMORPHISM. 



deep in Lancashire ; but the Cambrian and Silurian rocks in which 

 cleavage occurs must have been depressed twice as deep. 



From considerations of this kind we are led to admit that depth 

 in the earth that is, the heat, and pressure, and molecular action 

 favoured by depth, and of which depth is a measure is one of the 

 main agencies favourable to the generation of slaty cleavage in the 

 strata. Pressure is clearly necessary. For the direction of the planes 

 of cleavage is parallel to the great axis of movement in the district 

 which determines the strike of the rocks ; and Professor John Phillips's 

 researches, enlarged by the investigations of Mr. Sharpe, left no doubt 

 that the compression of the rocks is in the direction at right angles to 

 the cleavage planes. Mr. Sorby succeeded in producing cleavage struc- 

 ture by artificial pressure in clay originally quite destitute of it. Some 

 further illustrations were added by Dr. Tyndall, who used pressure to 

 develop cleavage in wax. Hence we arrive at the following general 

 view. A large area of country subsides parallel to a certain axis of 

 movement, is thrown into parallel folds, and is transferred to a hotter 

 and narrower space hotter as compared to the surface, narrower as 

 the chord is shorter than the arc. Lateral pressure operates on all the 

 strata ; heat more particularly on the argillaceous parts ; plates of 

 mica become scattered through these strata, and are by the pressure 

 made to assume positions which are not all parallel, but tend to 

 parallelism, and thus effectually cause fissility in the stone. 1 



Cleat. Though there is no slaty cleavage in the coal strata of the 

 northern counties, or indeed in Wales, there is a structure called 

 " cleat" in the substance of coal which is of the same order, quite as 

 regular and extensive, and due to as general a cause. This consists 

 in a series of parallel fissures, often very fine and numerous, which 

 cut across the strata of coal in planes nearly vertical to the strata, and 

 in directions seldom deviating much in the large area of a coal-field. 

 In the northern coal-fields this direction is N.N.W. and S.S.E., or 

 nearly so. It scarcely occurs except in the coal, is not affected by 

 faults, is not parallel to axes of movement, and varies in character 

 from bed to bed. This structure is of a very peculiar type in the 

 anthracite of Wales. 



Cleavage on Mont Blanc. In a survey of the structure of Mont 

 Blanc, 2 Mr. Sharpe traced no fewer than nine parallel axes of slaty 

 cleavage, crossing the gneissic, calcareous, and argillaceous strata, 

 which dip in various directions, a phenomenon analogous to 

 that observed by the same geologist in the country of the High- 

 lands. 3 



Molecular Metamorphism. Under this head we class the con- 

 version of earthy carbonate of lime into crystallised marble, which has 

 been effected naturally, by the proximity of igneous rocks, in many 

 places, as in Teesdale by the basalt or whin sill, in Raghlin by 

 basaltic dykes, and artificially by Sir James Hall in a heated gun- 

 barrel. On a large scale, saccharoid limestone is a great example, 

 proving the pervading influence of high heat through a mass of deep- 



1 Sorby, in Q. J. G. S., &c. 2 Geol. Proceedings, 1855. 3 Phil. Trans., 1855. 



