402 Rei\ 0. Fisher — On Cleavage and Distortion. 



for. If the plane of bedding lies so that the fossils become shortened, 

 they may be crumpled, and exhibit strijB parallel to the strike. 



While speaking of distortion, the greenish oval spots, which occur 

 on some purple slates, must not be passed over. It is perfectly 

 obvious that they are discoloured regions, and that the chemical 

 action has radiated from a foreign body, usually to be seen at or 

 near the centre. The first question is, whether they were formed 

 before or since the cleavage. Either is possible. If before, then 

 they have been distorted into their present form, and, if originally 

 spherical, they will give accurately the form of the ellipsoid of 

 distortion. If formed subsequently to cleavage, they may be ac- 

 counted for by the chemical influence spreading most readily along 

 the grain of the slate, and with greatest difficulty across its lamina3. 

 It is common to see exactly similar oval sjjots where the colour has 

 been discharged around nail-heads upon deal boards, which have 

 been stained to imitate oak and exposed to damp. The nail is in 

 this case the foreign body, and the spot is elongated in the direction 

 of the grain of the wood. It is difficult to see how the rock can 

 have been affected by local decomposition of the colouring matter, 

 unless subjected to the percolation of aerial water ; and this leads 

 me to incline tc the idea that these spots are comparatively recent. 

 Discoloured bands of bedding are in the same category. 



On studying Dr. Haughton's ingenious paper in the Phil. Mag. 

 for January, 1856, I was much perplexed at some of the results 

 which he had obtained. For instance, it seemed a very unac- 

 countable thing that the ellipsoid of distortion should commonly 

 come out a very flattened spheroid. This could only be explained 

 by an inconceivably great condensation of the material, with 

 scarcely any lateral movement. I think that the probable cause 

 of this anomaly is that Di-. Haughton has not taken into con- 

 sideration the alteration, in the angle between the original bedding 

 and the cleavage, produced by the shear. He calculates the form 

 of the ellipsoid by means of the distortion of a fossil out of its 

 known proportions as measured on the cleavage plane, and the 

 angles (0, 0') observed between the constant dip of the cleavage 

 and two different dips of the bedding, assuming that the bedding 

 has not been disturbed by the cleavage ; whereas it seems that 

 observation cannot determine the inclination of the original bedding 

 to the cleavage, because the bedding, as well as the fossils, has been 

 displaced. The formula which shows the amount of displacement 

 for a given shear will be given further on.^ 



To assist me in forming an estimate of the amount of condensation 

 which may have been produced by pressure during the formation of 

 a slate. Professor Liveing kindly examined for me a piece of the 

 Devonian argillaceous purple rock, taken at the depth of about 

 1000 feet, in the boring at Turnford, near Cheshunt ; and also a 

 fragment from a slab of purple slate. He found the Devonian rock 

 more like slate, than are ordinary clays, in its chemical composition. 

 It therefore appears to me to be a fair subject for comparison. Its 



1 Equation 3. 



