Rev, 0. Fisher — On Cleavage and Distortion. 401 



and it can be seen in Sorby's figure that the directions of the cracks 

 cut up the bent layers into voussoirs, just of the forms and in positions 

 which would be requisite to extend a fold already existing, into a 

 narrower one of sharper curvature. This applies to the folds which 

 point upwards as well as those which point downwards. The bending 

 and crowding of the cleavage surfaces, to accommodate themselves 

 between the folds of the less yielding rock, is analogous to what 

 may be seen under the microscope, where the laminte of the matrix 

 of a slate bend round a distorted harder granule. 



The principle just enunciated is capable of an application far 

 wider than can be now entered upon. It shows that all folding is 

 distorted by the action which has produced cleavage : and that, when 

 we see a fold in a cleaved region, we can no more assert that it is 

 of its original form, than that a fossil distorted by cleavage is so. 

 This will explain the shai'p folds dipping inwards on the flanks of a 

 mountain chain. It will also explain some cases, at least, of crumpling, 

 which may be but distorted ripple marks. And it is interesting to 

 see how closely the distorted section of ripple-mai'ked bedding, if 

 across the cleavage as given by the tesselated parallelogram, agrees 

 with that of a crumpled rock. 



Moreover, since this action affects the minute structure of the 

 rock, and leaves its impress upon microscopical sections, the mole- 

 cular movement, which in a suitable material produces cleavage, 

 may in such rocks as are not liable to take on that peculiar 

 condition, give rise to some analogous structure, which petrologists 

 might identify. 



The estimate hitherto made of distortion has been of that in a plane 

 at right angles to the strike. To estimate that in one parallel to it 

 (say on a bedding plane), we observe that every generating circle of 

 the sphere, perpendicular to the plane of the paper, has been changed 

 into its own corresponding ellipse. This ellipse will be of the same 

 breadth as the circle in the direction of the strike, but will be 

 elongated or shortened according to its position in dip. Its area 

 will not be equal to that of the circle, as was the case with the 

 vertical section of the ellipsoid. The result will be that the im- 

 pressions of fossils on the plane of bedding may be either elongated 

 or shortened in the dip, but will not be altered in the strike. They 

 will therefore in general cover a different area, greater or less, from 

 what they did before distortion ; while the apparent alteration of 

 their proportions in the direction of the strike will be only com- 

 parative. If we employ a divided square and a correspond ingl}'- 

 divided rectangular parallelogram, their lengths being taken pro- 

 portional to the radii of the circle and of the ellipse, parallel 

 respectively to the original and to the sheared bedding, it is easy to 

 delineate the distortion of any fossil ; and it will be found to agree 

 with that seen in nature. The comparison is best made in the case 

 of Trilobites. The same thing may also be readily done with 

 proportional compasses. When the bedding nearly coincides with 

 a cleavage plane, the fossils become much elongated, and so 

 enlarged, and thus the great size of the Tintagel fossils is accounted 



DECADE HI. — YOL. I. — NO. IX. 26 



