ANNIVEESAEY ADDEESS OF THE PEESIDEl^T. 



73 



Tarbet; only they are on a scale about -^\ as large, and therefore 

 the rock appears to have an imperfect cleavage throughout rather 

 than a iointed structure. Here, then, we have an imperfect clea- 

 vage due to the lateral yielding of a rock composed of material not 

 truly plastic in its ultimate constitution. 



Fig. 3. — Slate, Liskeard, with 

 minute contortions. 



Fig. 4. — Slate, LisJceard, with 

 planes of discontinuity. 



Pig. 5. — Slate, Shap, disturbed 



Yig, 6. — Slate, Llanheris, with 

 well-c 



The peculiar laminated structure of this Liskeard slate appears to 

 have favoured the production of this close-jointed structure, and 

 the joints are still further marked by the subsequent introduction 

 of black oxide of iron ; but in some cases, as in the pencil-slate of 

 Shap, the flakes of mica which constitute nearly the whole of the 

 rock have been thrown out of their original position in a similar 

 manner to the contorted material of the Liskeard slate, often with- 

 out the formation of definite contortions or joints, as shown some- 

 what diagramaticaUy by fig. 5, magnified about 200 linear. 



We thus arrive at a structure in which the flakes in each minute 

 broken portion do lie in the plane of stratification, but this plane 

 is so contorted and broken up, that, as seen in a section cut at 

 right angles to both bedding and cleavage, they are nearly equally 

 inclined at all azimuths. The amount of the lateral yielding of 

 the mass necessary to give rise to this structure is apparently 

 small compared with that which has certainly occurred in the case 

 of many well-cleaved slates ; but it clearly shows how the particles 

 may soon become so arranged that the further yielding of the rock 

 would totally efface all evidence of the first stages of the process, 

 and cause the constituent laminse to become approximately parallel 

 to one plane, as shown, somewhat diagramaticaUy, by fig. 6, magni- 

 fied about 200 linear. 



