40 Prof. Tyndall on the Cleavage of 



tion to the question ; right or wrong a theory thus thoughtfully 

 uttered has its value ; it is a dynamic power which operates 

 against intellectual stagnation; and even by provoking oppo- 

 sition is eventually of service to the cause of truth. It would 

 however have been remarkable if, among the ranks of geolo- 

 gists themselves, men were not found to seek an explanation of 

 the phsenomena in question, which involved a less hardy spring 

 on the part of the speculative faculty than the view to which I 

 have just referred. 



The first step in an inquiry of this kind is to put oneself into 

 contact with nature, to seek facts. This has been done, and the 

 labours of Sharpe (the late President of the Geological Society, 

 who, to the loss of science and the sorrow of all who knew him, 

 has so suddenly been taken away from us), Sorby and others 

 have furnished us with a body of evidence which reveals to us 

 certain important physical phsenomena, associated with the ap- 

 pearance of slaty cleavage, if they have not produced it : the 

 nature of this evidence we will now proceed to consider. 



Fossil shells are found in these slate-rocks. I have here 

 several specimens of such shells, occupying various positions with 

 regard to the cleavage planes. They are squeezed, distorted 

 and crushed. In some cases a flattening of the convex shell 

 occurs, in others the valves are pressed by a force which acted 

 in the plane of their junction, but in all cases the distortion 

 is such as leads to the inference that the rock which contains 

 these shells has been subjected to enormous pressure in a direc- 

 tion at right angles to the planes of cleavage ; the shells are all 

 flattened and spread out upon these planes. I hold in my hand 

 a fossil trilobite of normal proportions. Here is a series of fos- 

 sils of the same creature which have suffered distortion. Some 

 have lain across, some along, and some oblique to the cleavage 

 of the slate in which they are found; in all cases the nature 

 of the distortion is such as required for its production a com- 

 pressing force acting at right angles to the planes of cleavage. 

 As the creatures lay in the mud in the manner indicated, the 

 jaws of a gigantic vice appear to have closed upon them and 

 squeezed them into the shape you see. As further evidence of 

 the exertion of pressure, let me introduce to your notice a case 

 of contortion which has been adduced by Mr. Sorby. The bed- 

 ding of the rock shown in this figure was once horizontal ; at 

 A we have a deep layer of mud, and at mn a layer of compara- 

 tively unyielding gritty material ; below that again, at B, we 

 have another layer of the fine mud of which slates are formed. 

 This mass cleaves along the shading lines of the diagram : 

 but look at the shape of the intermediate bed : it is contorted 

 into a serpentine form, and leads irresistibly to the conclu- 



