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LIU. A Geological Sketch of a Portion of the Granite District 



near Penryn, referred to in the preceding Paper, By 



R. W. Fox, Esq* 



[With a Map : Plate IV.] 



Explanation of the Map. 



T^HE unshaded part is intended to show the direction of 



•*" the range of hard or compact granite near Penryn, which 



is nearly N.E. and S.W. Its length about 5 or 6 miles, and 



its breadth 1 to l{ mile. 



The dotted part represents a coarser granite, less compact, 

 and often friable near the surface. 



The granitic district extends many miles towards the north- 

 west, and includes, as there is reason to believe, other ranges 

 of compact granite nearly parallel to the above. 



The shaded part represents clay-slate, or " killas," resting 

 on the granite. 



Some of the quarries in which the cleavage has been ex- 

 amined, are marked thus, + ; and it appears that the average 

 direction of the vertical cleavage is nearly N.N.W. and 

 S.S.E. There seems to be a remarkable approximation to 

 uniformity in this respect, although not so decided as in the 

 horizontal, or almost horizontal cleavage. 



The Main Rock, and some other very large rocks, which are 

 above the surface, seem to correspond with the lines of cleavage; 

 thus affording strong evidence of their being in their original 

 position. 



In reference to the statement in the preceding paper relative 

 to the cleavage of granite near Penryn, it may be proper for 

 me to say, that the horizontal cleavage, or " capping" as it is 

 termed, is by no means confined to the granite in question; but 

 on the contrary, it seems to be a characteristic of that rock in 

 different parts of Cornwall; as I find it has been observed at 

 Kit-hill, and other places near Callington, at Rough- tor near 

 Camelford, and at Carnmarth near Redruth. In these di- 

 stricts likewise, the granite possesses natural joints in a hori- 

 zontal direction, or nearly so, although they are often almost 

 imperceptible, except where they have been enlarged by the 

 action of the weather. Besides these joints, there are other 

 similar ones at right angles to them ; but I am not prepared 

 to state whether, as in the Penryn granite, they have any 

 tendency to uniformity in their bearing. In all the cases al- 

 luded to, there appears however to be a correspondence be- 

 tween the " capping," and the nearly horizontal joints ; on 

 which account it seems reasonable to infer, that the similar 

 joints which abound in the granite at and near the Land's End, 

 * Communicated by the Author. 



