116 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 1, 



of the secondary cleavage, but the situation of the quarries near Shap 

 is very unfavourable to such an object, and I could not learn of any 

 other similar rock in the district. The quarries are worked on low 

 moors where very little of the rock is exposed for several miles round, 

 and little can be seen of its relations to the neighbouring beds. 

 Judging from analogy with the supposed cause of the principal 

 cleavage, it may be conjectured that the secondary cleavage is due to 

 a second compression which the rock has undergone in a different di- 

 rection from the first. This is in harmony with the position of the 

 Shap pencil-rock, between two great masses of erupted rocks of dif- 

 ferent dates, the granite of Wastdale Head on the south, and the 

 porphyritic bands on the north ; and is confirmed by the numerous 

 joints observed in the quarries, which show that the rock has under- 

 gone great and complicated pressure. 



Cleavage not connected with crystallization. — In the rocks ex- 

 amined, I could find no connection between crystallization and clea- 

 vage ; on the contrary, everything tended to show that the cleavage 

 has been produced by causes solely mechanical. In many of the 

 beds of slate there are cubical crystals of iron pyrites scattered through 

 the rock without any reference to the direction of the cleavage : va- 

 rious rhomboidal crystals of different minerals also occur in slate, but 

 I have seen no instance of their sides being based on the cleavage- 

 planes, or of their having any definite position in the rock. In the 

 chiastolite slate of Skiddaw, the long prisms of chiastolite traverse 

 the rock in all directions, without showing any preference either for 

 the planes of bedding or cleavage. 



But the most conclusive evidence is found in the altered slates 

 resting on the igneous rocks, in which the original mechanical struc- 

 ture of the rock is seen in combination with a considerable degree of 

 crystallization. In these rocks, which are common in the green 

 slate district, all the mechanical portions of the rock appear to have 

 been compressed between the planes of cleavage, but the crystalline 

 portions are quite independent of the cleavage, both in position and 

 form. 



Moreover, in tracing the gradual change of character as we pass 

 from true slates, through the metamorphic rocks to those of decidedly 

 igneous origin, we always find the cleavage more developed in pro- 

 portion as the rock is least crystalline. In the best roofing-slate, 

 where the cleavage is strongest, there is no trace of crystallization : 

 in the metamorphic slates the cleavage is fainter in proportion as the 

 crystals in them are more abundant ; and on reaching a thoroughly 

 crystalline rock, we lose the cleavage entirely. This alone is a sufii- 

 cient proof that the cleavage had an origin unconnected with crystal- 

 lization. 



There are apparent exceptions to the above remarks in the thin 

 laminse of talc and mica which are found lying along the cleavage- 

 planes of certain slates ; for although those laminae have no definite 

 form, their segregation from the mass of the rock is undoubtedly a 

 process approaching to crystallization. Probably the reason that 

 they have been formed on the planes of cleavage is, that the rock 



