Page:VCH Cornwall 1.djvu/77

 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. 29