232 



Prof. T. G. Bonney. 



The " Blue Ground " and Associated Bocks. 



Two areas of cliamantiferous rock are now being worked at the 

 Newlands Mines. The shape of the one which supplied most of the 

 specimens described in this paper is irregular, and, so far as I know, 

 exceptional. Its outline at the surface may be roughly compared to a 

 rounded triangle into the base of which the point of a rather short 

 shuttle is thrust, the greatest breadth of the two being about equal. 

 Exploratory workings at a depth of 300 feet show that the former 

 area rather quickly narrows, and the latter terminates in clefts. The 

 " blue ground," in fact, appears to fill a fissure, broadening in two 

 places to vents, which has been traced for some distance underground 

 southwards from the principal mass of diamantiferous rock, as repre- 

 sented in the annexed section, where the latter is dotted. 



Fig. 3. 



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

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

 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 walls of diabase, farther to the south, turn off rather sharply to 

 east and west. In the interval, about 12 feet in width, between them, 

 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 Cornwall. 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 



