300 Prof, Tyndall, on the Comparative View of [June 6, 



the mass a single plane of crystalline cleavage. With reference to 

 this hypothesis, I will only say that it is a bold stretch of analogies : 

 but still it has done good service ; it has drawn attention to the 

 question ; right or wrong a theory thus thoughtfully uttered has its 

 value ; it is a dynamic power wliich operates against intellectual 

 stagnation ; and even by provoking opposition is eventually of ser- 

 vice to the cause of truth. It would however have been remarkable 

 if, among the ranks of geologists themselves, men were not found 

 to seek an explanation of the phaenomena in question, which 

 involved a less hardy spring on the part of the speculative faculty 

 than the view to which 1 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 fur- 

 nished us with a body of evidence which reveals to us certain 

 important physical phaenomena, associated with the appearance 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 direction 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 fossils 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 compressing force acting at right angles to the planes 

 of cleavage. As the creatures lay in the mud in the manner in- 

 dicated, 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 bedding of the rock shown in this figure was once horizontal ; 

 at A we have a deep layer of mud, and at m w 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 conclusion that the 

 mass has been pressed together at right angles to the planes of 



