chip. vn. Junction of Granite and Clay Slate. 167 



for a separate formation. I was guided by Dr. Andrew 

 Smith to a fine junction at Green Point between the 

 granite and clay-slate : the latter at the distance of a 

 quarter of a mile from the spot, where the granite 

 appears on the beach (though, probably, the granite is 

 much nearer underground), becomes slightly more com- 

 pact and crystalline. At a less distance, some of the 

 beds of clay- slate are of a homogeneous texture, and 

 obscurely striped with different zones of colour, whilst 

 others are obscurely spotted. Within a hundred yards 

 of the first vein of granite, the clay-slate consists of 

 several varieties ; some compact with a tinge of purple, 

 others glistening with numerous minute scales of mica 

 and imperfectly crystallised feldspar ; some obscurely 

 granular, others porphyritic with small, elongated spots of 

 a soft white mineral, which being easily corroded, gives 

 to this variety a vesicular appearance. Close to the 

 granite, the clay- slate is changed into a dark-coloured, 

 laminated rock, having a granular fracture, which is 

 due to imperfect crystals of feldspar, coated by minute, 

 brilliant, scales of mica. 



The actual junction between the granitic and clay- 

 slate districts extends over a width of about 200 yards, 

 and consists of irregular masses and of numerous dikes 

 of granite, entangled and surrounded by the clay-slate : 

 most of the dikes range in a NW. and SE. line, 

 parallel to the cleavage of the slate. As we leave the 

 junction, thin beds, and lastly, mere films of the altered 

 clay-slate are seen, quite isolated, as if floating, in the 

 coarsely-crystallised granite ; but although completely 

 detached, they all retain traces of the uniform NW. 

 and SE. cleavage. This fact has been observed in 

 other similar cases, and has been advanced by some 

 eminent geologists, 1 as a great difficulty on the ordinary 



1 See M. Keilhau's ' Theory on Granite,' translated in the 'Edin- 

 burgh New Philosophical Journal,' vol. xxiv. p, 402. 



