Page:A Treatise on Geology, volume 2.djvu/135

 CHAP. VII. but it does not yet appear that any change of dimensions can be shown in the direction of the strike of the cleavage. No one can doubt that here we have indications of exact mechanical laws, operating on masses of matter, so regularly as to emulate the results of crystalline force on the molecules. But the latter force is free to arrange molecules singly by polar attractions, the former is constrained to obey certain axes in the mass.

Cleavage is remarkably developed in some districts which are formed upon one or more axes of anticlinal elevation and synclinal depression: for example, in Cumbria, Wales, and Devonshire. In each of these cases the fact is patent that the cleavage runs for 20 or 30 miles in one continuous direction, which is observed by all the cleavage planes over a considerable breadth of country. This direction is parallel to the great axes of movement in that district, almost exactly so on a great scale, though deviating slightly from the strike of the beds in particular places, especially when the strata are in any degree twisted.

The cleavage is in fact but little, if at all, affected by small irregular twists of the beds, and is, on the whole, more regular in its strike than they are. It is related to the great axes, not to the local bedding. May we from this infer that the general pressure on the axes of movement has been a determining cause of the new structures parallel to these axes?

Another thing is remarkable. The cleavage is less frequently vertical than inclined at a high angle, say 70°. It is also found at 45°, 30°, 20°, and even at much lower angles. Most frequently, when the strata are much inclined, the cleavage is inclined still more; but this has exceptions.

As the cleavage strike is not really dependent on the strike of the beds at a particular place, so is its dip not really dependent on the dip of the strata there: there may be more than one anticlinal and synclinal of strata (besides minor folds) and yet only one cleavage system.