GEOLOGY 



liquid bubbles that move about in tiny cavities, and have resulted from 

 the condensation of steam which such cavities formerly imprisoned. 



The system of jointing pertaining to the Cam Menelez granite bears 

 a definite relation to the crystalline arrangement. There are three well 

 defined joint planes ; one set of vertical joints, having a prevalent direc- 

 tion of N.N.W., is crossed by another vertical set at right angles. These 

 two systems, in conjunction with a third set more or less horizontal, 

 divide the whole rock into a set of rough prismatic segments. Such 

 vertical columns of rectangular blocks are admirably displayed by the 

 weathering of the granite in the rugged coast scenery of the Land's 

 End. The rock cleaves most readily along planes parallel to the hori- 

 zontal joints ; the next easiest cleaving plane is that parallel to the 

 N.N.W. joints, while the rock cleaves most irregularly parallel to the 

 E.N.E. joints. Not only is there a close connection between the major 

 joints and the grain of the rock, but the grain itself is dependent on the 

 internal mineral arrangement, and all these phenomena are closely related. 

 The internal crystalline arrangement appears to consist first in a tendency 

 for the mica to lie with its basal planes horizontal ; secondly, in a dispo- 

 sition of the felspars, both as constituents of the matrix and as porphyritic 

 individuals, to rest with their flat sides in a similar position ; and thirdly, 

 in the orientation of the felspars with their long axes parallel to the 

 N.N.W. (cleaving way) joints. The first and second of these structures 

 probably explains the proneness to cleave parallel to the horizontal joints ; 

 while the third seems to show why the rock tends to cleave in planes 

 parallel to the cleaving-way joints. 



The uniformity in these structural features is not only diffused over 

 the whole granite mass, but is common to the whole of the post-Car- 

 boniferous granites of Cornwall. While the granite presents only 

 exceptionally a marked foliated appearance, it has evidently undergone 

 throughout a rude and initial stage of foliation, whereby its component 

 minerals have been forced to rearrange themselves in a definite direction 

 so as to acquire a cleavage. Judging from the data furnished by the 

 district between Gerrans Bay and Truro, there are reasons for believing 

 that the sediments owe their deformation to stresses acting along a 

 N.N.W. direction which agrees with one of the main cleaving directions 

 of the granite. There are strong grounds therefore for assuming that 

 the granite irruptions and the earth movements are not only closely 

 related, but that the movements had not ceased to operate before the final 

 solidification of the granite. This assumption is still further supported 

 by the evidence of two sills of foliated granite that flank the margin of 

 the Cam Menelez mass near Penryn, and by the foliated or gneissose 

 character of the marginal granite at Kennal Vale near Ponsanooth, the 

 foliation in both instances coinciding with the cleavage of the adjacent 

 slates. A small intrusion of tourmaline-muscovite granite near Truthall 

 is so sheared that the quartz and felspar are often granulitic with 

 muscovite bent and dragged out. 



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