514 MK. A. HAKKER ON ROCKS 



fit into one another can scarcely be expluiued except on the sup- 

 position that they have been recrystallized under the influence of 

 mechanically produced stress. 



These metamorphosed slates of BroAvnber contain pseudomorphs of 

 limonite, about a tenth of an inch in diameter, evidently replacing 

 cubes of pyrites. The pyrites has been formed in situ, for occasional 

 grains of quartz, &c. are enclosed. Moreover, its decomposition 

 has been subsequent to the crushing of the rock, for the cubes are 

 not sensibly distorted. Indeed, the movement of the rock about 

 the pyrites-crystals has been such as to leave vacant spaces, after- 

 wards filled by secondary quartz. This quartz has a rather fibrous 

 structure, and is arranged at right angles to the faces of the pyrites 

 cubes. It is found only on those parts of the cubes where the 

 pressure would be relieved by the flowing movement of the rock- 

 mass, and the phenomena are precisely similar to those which I 

 have elsewhere described as common in pyritous slates *. 



As to modifications produced by thermal agency, a few words will 

 sufiice. The Skiddaw Slates show some degree of metamorphism 

 near their contact with the Cuns Fell diabase. A slice in Prof, . 

 Nicholson's collection is a well-laminated rock, in which numerous 

 minute grains of clastic quartz are mingled with the argillaceous 

 material. It is marked throughout with irregularly ovoid spots, 

 one-fiftieth to one-hundredth of an inch in diameter. Along 

 certain bands these spots are merely clear patches due to the 

 dusty (carbonaceous ?) matter having been expelled, to collect just 

 beyond the margin. In other bands the clear spots thus left behave 

 optically in a difi'erent way from the surrounding ground, being 

 mostly dark between crossed nicols. The quartz-sand occurs 

 indifferently inside and outside the spots, and the grains have lost 

 nothing of their sharpness of outline. 



Near the large lamprophyre dyke in Dry Sike, again, the Skiddaw 

 Slates appear highly metamorphosed, being converted into a very- 

 compact black rock with a certain degree of lustre and a conchoidal 

 fracture, like some varieties of hornfeh. A slice of this rock, which 

 is rather a microscojjic grit than a true slate, shows as the chief 

 metam Orphic product a rather obscure chloritic mineral. The 

 numerous minute quartz-grains retain their angular outline [912]. 



Among the lavas occurring in the Skiddaw Slate group, an 

 interesting rock was collected by Mr. Marr in the stream north- 

 west of Master Sike [920]. It is an andesite consisting essentially 

 of an isotropic base crowded with ver}- minute felspar-microliths. 

 These only occasionally show any parallel arrangement, although a 

 streaky fluxion-structure is seen in the mass as a whole. There 

 are a few small porj^hyritic felspars with good outlines. No augite 

 is recognizable, though its former presence is probably indicated by 

 the pale delessite-like substance filling some small ovoid vesicles in 

 the rock. The interesting point is a vesicle about a twelfth of an 

 inch in length, with a complex structure recalling in some respects 



* Geol. Mag. (1889) pp. 396, 397. 



