Rev. 0. Fisher— On Cleavage and Distortion. 



399 



One mode of representing the distortion by shearing in a plane 

 perpendicular to the strike (the plane of dip) is as follows. Draw 

 a number of lines parallel to the shear, dividing the circumference of 

 the circle of Fig. 1 into equal small arcs, and produce these lines to 

 cut the ellipse. The points where they cut the ellipse will be the 

 places, in which the corresponding points of the circle will be found 

 after being sheared. (It is easy to conceive an analogous con- 

 struction if the circle has been compressed as well as sheared.) 

 And points along the radii of the circle will take up their positions 

 along these radii of the ellipse, as defined by their intersections with 

 the parallel lines. We may then, if we please, draw our fossil, or 

 other object, within the circle ; and draw a distorted figure within 

 the ellipse, guiding the pencil by corresponding intersections of the 

 parallels with the radii in the circle and in the ellipse respectively. 

 But for practical purposes it is more convenient to use the parallelo- 

 gram into which a square is distorted, as in the figure below. 

 By dividing up the square into a number of small squares, and the 

 parallelogram into the corresponding small parallelograms, it is then 

 very easy to draw the outline of any object within the square, and 

 to represent its distorted form within the tessellated parallelogram. 



Fig. 3. 



Shear 



Shear ^^%-- 



^^^^ 



Sectioist of a Splrifer distorted by shearing. 



Part of Sharpb's Diagram (Fig. 15) showing the distortion of a Spirifer 

 BY cleavage at South Pethertvin.i 



The outline of a Spirifer given in the diagram has been taken 

 along a section slightly inclined to the line of the hinge, so as to 

 give a distorted form analogous to that in Sharpe's figure. On com- 

 paring the two figures, it can be easily seen that, on a chance 

 assumption of the position of the fossil and of the amount of the 

 shear, the distortion agrees very well with that observed by Sharpe. 

 Some allowance may fairly be claimed for want of closer agreement 

 1 Quart. Journ. Geol. See. vol. iii. 1846. 



