part 2] GEOLOGT or THE MELDON VALLEYS. 83 



drive a trench some distance to the north in the hope of striking- 

 limestone, but without success. 



The black rock bordering the Meldon limestone may be found 

 in large quantity in the quarry- waste ; it is nearer absolute opacity 

 than any mineral, other than coal or metallic ores, that is known 

 to me, and it lias most of the bad qualities that a rock can show 

 as a material for section-cutting. 



It is, however, possible to grind small fragments thin enough for 

 examination. The black pigment is then seen to consist of minute 

 irregular grains, either discrete or in somewhat linear aggregates, 

 and marking out an ill-defined striation. The other minerals are 

 colourless (with one exception) and minutely granular : for the 

 greater part their double refraction is about equal to that of quartz, 

 but some very small lath-forms have a rather higher double refrac- 

 tion. These forms can be isolated from the rotten bands inter- 

 calated in the strata : they are frequently clouded with the black- 

 pigment, the direction of elongation is positive, the}' have straight 

 extinction, their refraction about equals that of Canada balsam, 

 and their double refraction exceeds that of quartz by a little ; 

 boiling hydrochloric acid does not affect them. The exception 

 above referred to is iron pyrites, grains of which may be seen by 

 the naked eye ; many more are shown by a lens, and much is 

 microscopic. In minute form the rock contains a great deal of 

 pyrite, which, like the black pigment, in part aggregates in short 

 groups, all of which have their longer axes parallel. 



Prolonged ignition of the rock largely discharges the black 

 pigment from the immediate surface, and makes the colour a 

 rusty black in consequence of the alteration of the pyrites. The 

 pigment appears to be carbon. 



The rotten bands, spoken of above as clay, are not really clays — 

 their adhesion and plasticity alike are of the slightest. Chemical 

 tests show the presence of a trace of manganese. If the material 

 is boiled in hydrochloric acid, it yields a solution containing iron, 

 a little alumina and lime, and some magnesia. 



The sections of black rock above described bear the number 

 LXXVI, S.E. 1 c, and the decomposed rock was taken from the 

 eastern bank of the West Okeinent. opposite the Limestone Quarry. 



LXXVI, S.E. 11, on the northern bank of the Redaven, yields a 

 very similar black rock, but of slightly liner texture. This, while 

 near akin to 1 c, is not identical. It contains much pyrite in 

 very small form, and some joint-faces have thin linings of pyrite. 

 The rock is less opaque, and sections are much more easily made 

 from it. In section it shows bands of coarse and of hue texture, 

 of narrow width, the succession being- coarse, hand of tine OS mm. 

 wide, band of coarse 1"6 mm. wide, band of fine l'G mm., thence 

 grading into coarse without any abrupt junction. A striation 

 due to the pigment crosses these hands, passing continuously from 

 one to the other and making an angle of 34° with their direction. 

 a feature difficult to account for. 



As distinct from the two rocks last described, which have only 

 Q. J. G. S. No. 29S. n 



