Vol. 59.] THE GR1NITE AND GREISEN OF CLIGGA HEAD. 145 



for, eveD should the spotting be found to be existent outside the 

 aureole mapped in fig. 1 (p. 144), the bleaching alone, which can be 

 traced in all its gradations to Pen-a-gader on the south-west and 

 Chapel Eock on the north-east, allows of an aureole of the same form 

 (which is the essential point in dealing with the Cligga granite) if not 

 of the same extent. 



To return to the granite : the form of the outcrop is elongated, the 

 axis trending due north and south, and the section shows innumerable 

 divisional planes trending about 20° north of east. In the sea, west of 

 the cliff- section, are a number of skerries, chiefly of granite, but those 

 opposite the southern end of the granite are of altered killas. Now, 

 it will be seen by referring to the map (fig. 1), that from the junction 

 with the killas on the south the granite-outcrop must have trended sea- 

 ward to the north-west. Again, if the form of the divisional planes 

 at the extreme south is examined, it will be found that the granite 

 rests upon a steep wall of killas. This also is the case close by on the 

 east ; but there is this difference : the junction on the south is a 

 plane-surface, while that on the east forms a shallow cup with the con- 

 cavity pointing to the west. The only means of tracing the eastern 

 junction northward is by old mine-debris on the Factory grounds, 

 which just suffices to show that its direction is roughly parallel to 

 the cliff-section. Nor can any better evidence be obtained from the 

 slopes of the cliff facing northward, though it is possible to see the 

 junction in the lower part of the cliff from a boat. It proves to be 

 vertical, and is clearly defined, owing to the sea having driven a 

 narrow chasm some feet into the face of the cliff along the plane of 

 contact. The fact of this junction being vertical lends colour to the 

 miners' supposition, quoted by De la Beche, 1 that the granite has 

 the habit of a dyke. But my friend, Capt. Turner, placed at my 

 disposal a mining report dated 1857 & 1858, discussing the advisa- 

 bility of re-opening the Perran United Copper Sett, formerly worked 

 on the ground now occupied by the Explosives Factory, in which it 

 is stated by Capt. John R. Pill that, apart from the exposure of the 

 Cligga-Head granite, the main body of the granite comes near the 

 surface at a point at sea-level three-quarters of a mile to the east, 

 a fact unknown in De la Beche's time, and one that throws an 

 entirely new light on the situation. In the first place it immediately 

 explains the significance of the peculiar form of the metamorphic 

 aureole, in relation both to the position and to the size of the visible 

 outcrop ; and in the second place, judging from this evidence, there 

 seems to be no reason to doubt that this is a faulted junction, the 

 downthrow of the fault being to the east. Yet that this fault, 

 which agrees roughly in direction with the great ' cross-courses,' 

 does not affect the whole of the eastern outcrop is evident from a 

 study of the divisional planes at the extreme south of the granite- 

 mass. It will be objected to this view by anyone who has seen 

 Conybeare's figure " that this fault must necessarily break the course 



1 ' Geol. Rep. on Cornwall, Devon, & W. Somerset ' 1839, p. 162. 



2 Trans. G-eol. Soc. vol. iv (1817) pi. xxiii. 



