﻿126 MR. E. H. EASTALL OX THE [May IpIO,. 



Type I.— The Black Slate. 



The rock here described as black slate represents the finest type 

 of sediment, deposited in fairly deep water, so that it is comparatively 

 free from gritty particles. As a result of the dynamic metamor- 

 phism at an earlier date a certain amount of mineralogical change 

 was induced, of which the most important feature was in the 

 majority of cases the formation of secondary mica in minute flakes 

 but different specimens show a considerable amount of variation. 



A typical example of such a rock is seen under the microscope to 

 consist, for the most part, of a felted aggregate of small flakes of 

 colourless mica, with occasional minute grains of quartz, and a con- 

 siderable amount of black material, which appears to include both 

 carbon and iron ores ; in some specimens iron pyrites is abundantly 

 developed. There is also commonly present in large quantities a 

 mineral the exact nature of which is still somewhat uncertain : it 

 occurs in very minute, long, slender prisms, not well terminated, 

 and often showing a cross-fracture somewhat like that of apatite. 

 It is nearly colourless, or with a very faint greenish tinge, and 

 perhaps exhibits slight pleochroism, although, owing to the minute 

 size of the crystals and the paleness of the colour, this is difficult to 

 determine with certainty. The strongest absorption appears to be 

 parallel to the long axis. The refractive index is high and the 

 double refraction very weak, giving steel-blue or grey of the first 

 order. Most individuals seem to be simple twins, with a very narrow 

 extinction-angle. It is optically negative. This mineral seems to 

 be more nearly related to ottrelite than to anything else, and it 

 is not improbable that the latter should occur in these rocks as a 

 result of the dynamic metamorphism. 



(a) The outer spotted zone. — In some localities, though not 

 everywhere, the first sign of alteration seen on approaching the 

 nietamorphic aureole is the development on the cleavage-faces of 

 the slate of indistinct whitish or silvery spots. Except for the 

 presence of these spots, the slate is entirely unchanged. They seem 

 to develop most freely in the very black carbonaceous slates, and are 

 absent where the beds are at all gritty ; but they occur also in the 

 grey flag series. The outer spotted zone appears to be very narrow, 

 and sometimes extends over only a few yards, so far as can be 

 judged, since it passes gradually on the one hand into the unaltered 

 slates, and on the inner side crystals of chiastolite begin to appear 

 along with the spots. In one or two places, however, where the form 

 of the ground is especially favourable, it covers a large area. The 

 best instance of this is on Grey Crags, on the western face of Skiddaw 

 below the Low Man, where the outer spotted zone covers nearly 

 1500 feet of vertical height. This is to be accounted for by the 

 fact that the outer boundary of the aureole very nearly coincides 

 with the slope of the ground (see fig. 1, p. 127). 



The spots are whitish or slightly silvery in appearance, and either- 



