Page:A Treatise on Geology, volume 1.djvu/81

 CHAP. IV. by the following diagram, in which the breadth of shade in any direction corresponds to the number of fissures observed. Thus, in the direction N.N.W. and S.S.E., and E.N.E. and W.S.W., and about these lines, a greater number of fissures occur, thus producing two principal systems of divisions in the rocks at right angles to each other; while in the lines N.E. by N. and N.W. by W., and about these lines, few or no fissures have been noticed. Thus there are positive and negative axes of frequency and rarity of the fissures situated at right angles to each other respectively. The same result of predominance of fissures in N.N.W. directions is found to obtain in Derbyshire (Hopkins), in Cornwall (De la Beche), and in some districts of Ireland (Griffith).



The effect of these fissures in causing lines of weakness of the rocks may be understood from the diagram: the breadth occupied in each radius being proportioned