422 ME. J. PAEKINSOtf ON THE GEOLOGY OP [Aug. I903, 



a detrital origin ; in other words, it has doubtless formed in place. 

 Probably the quartz and carbonates were deposited by water, the 

 corrosion of the calcite indicating that, of the two, the water charged 

 with silica was the later. The presence of tourmaline also points 

 to hydrothermal agents. Possibly the magnetite was leached out 

 of a rock similar to that which constitutes the ' lenticles,' and con- 

 centrated elsewhere, crystallizing as it was deposited. 



That iron is present in some quantity in the neighbourhood is 

 shown by a thin section cut from a slaty-looking rock found some 

 2 yards away. Here we have some fairly well-formed crystals of mag- 

 netite, while the slide is thickly (though irregularly) powdered with 

 dust of the same mineral, possibly specular iron, and some ilmenite. 

 The rest of the rock consists of green chlorite, very inert with either 

 or both of the nicols ; quartz ; disseminated carbonates, associated 

 often with the quartz ; and some flakes of mica. A few small and 

 dusty grains may possibly represent a felspar. The rock is con- 

 ceivably a highly decomposed and altered diabase or dolerite. 



Two specimens underlying the limestone-band above described, 

 between the east side of the Rocky Valley and Trewethet Gut, 

 exemplify the passage of the Volcanic Series into the Lower Blue- 

 Black Slates. 



The first is a dull greenish-grey rock, spangled with bronze- 

 coloured plates of mica and speckled with minute black crystals. 

 On surfaces at right angles to the foliation calcite can be discerned. 

 In a thin section the dusty crystals of this mineral are conspicuous, 

 and are arranged, as in previous examples, in lenticular patches 

 resembling amygdaloids. Much indefinite, or, rather fibrous grey 

 material makes up the rest of the section, together with small 

 subangular grains of quartz and carbonates, films of white mica, and 

 larger crystals of a green variety of the last-named mineral. The 

 rock gives some evidence of crush followed by a revival in crystalli- 

 zation of the green mica. 



The second specimen is much more slate-like in appearance. In 

 colour it is bluish-grey with rusty spots, sufficiently soft to be marked 

 by the nail, and containing stumpy crystals of a black mineral. 

 Carbonates are absent, and the slide shows a rough banding, parts 

 composed of chloritic or micaceous films alternating with gritty layers. 



Intertwined with the subangular quartz-grains of the latter are 

 small flakes of greenish mica, averaging -0012 inch in length. The 

 black crystals of the hand-specimen are crystals of mica larger than 

 those mentioned, green in colour through alteration and formed 

 in situ, commonly athwart the foliation. The largest included in 

 the section is -05 inch in length. Haematite (probably) forming 

 irregular grains, possibly a little specular iron, and locally rusty 

 staining represent the iron-minerals. 



The Hallwell-Cottage Beds. 



Microscopically the Hallwell-Cottage Beds consist of closely- 

 interwoven flakes of sericite, in which unorientated crystals 

 (brightly polarizing) are thickly scattered. 



