Professor T. G. Bonneij — Tarent-roch of the Diamond. 317 



southwards from the principal mass of diamantiferous rock, as 

 represented in the annexed section. 



II 



III 



IV 



YI YII VI IV VI IV VI IV 



<r 



12 ft. 



-> 



An igneous rock occurs on either side. It is compact, a greenish- 

 grey in colour, not unlike some of the less acid Welsh felstones. 

 Under the microscope it is found to be much affected by secondary 

 mineral changes ; the iron oxides alone being in good preservation. 

 A few small crystals of decomposed felspar are scattered in a yet 

 more decomposed matrix, of which the minor details are uninteresting. 

 The rock may be classed with the compact, rather felspathic, diabases. 

 These, farther to the south, turn off rather sharply to east and west. 



In the interval, about 12 feet in width, between walls of this 

 diabase, ribs of the * blue ' and a mudstone alternate, the thickest 

 one of the former being from 3 to 4 feet in width, and the inner 

 part of it is in better preservation than the outer. Specimens have 

 been examined from the heart of the mass (vii), a part outside 

 it (vi), and the exterior portion (v). The first (vii) in texture, 

 hardness, and colour reminds me a little of the dark serpentine 

 found north of Cadgwith, in Coi'nwall. In this matrix roundish 

 spots occur, some darker than it, others a yellow-green colour, 

 besides a few angular whitish spots. The block is traversed by two 

 or three thin calcareous veins. Specimen (vi), while generallj^ 

 similar, is more decomposed, and apparently contains some fragments 

 of shale. Specimen (v) has a stratified aspect, being a dull grey, 

 faintly mottled rock, with streaky, dark, rather carbonaceous-looking 

 bands ; the origin being doubtful, till it is seen under the 

 microscope. A fourth specimen (iii) shows the mudstone traversed 

 by a vein of rather pale-coloured decomposed ' blue,' not exceeding 

 an inch in thickness. A fifth (ii) is from near the diabase on the 

 western side, a dark compact rock, faintlj'^ mottled, here and there 

 presenting a slight resemblance to a ' blue ' traversed by thin veins 

 of a carbonate ; and sixth (iv) from a like position on the opposite 

 side is a generally similar rock, but with wider veins filled with 

 more coarsely crystalline calcite. The last specimen represents the 

 ' blue ' in the ' neck,' a few yards to the north and at the same 

 level (300 feet). This, inferior in preservation to the first-named, 



