164 J. A. PHILLIPS ON THE SO-CALLED 



Water [ n yS rometric 

 Water \ combined ,. 



I. II. 



-68 -58 



3-20 3-14 



Silica 63-45 63-37 



Phosphoric anhydride , , trace trace 



Alumina 18-33 18-31 



Ferric oxide , 2-68 2-63 



„ persulphide traces traces 



Ferrous oxide 3-66 3-60 



Manganous oxide trace trace 



Lime -43 -59 



Magnesia 3-00 2-92 



Potassa 3-73 4-00 



Soda -78 -65 



99-94 99-79 



This is a grey killas, of more than usual hardness, and of which 

 some of the transverse headings are stained of a reddish-brown 

 colour by hydrated ferric oxide, while others are coated with 

 chlorite. 



Under the microscope it is found to consist of a nearly colour- 

 less base, enclosing a little viridite with mica, and numerous dis- 

 tinct fragments of quartz, together with a few aggregations of a 

 granular mineral, having the appearance of minute garnets. These 

 groups vary from lu 1 (J0 to ^J^ inch in diameter, and are sometimes 

 darkened and rendered opaque by. the liberation of hydrated ferric 

 oxide ; when viewed between crossed nicols the base presents the 

 usual coloured mosaic characteristic of such rocks. 



Upon a hill a little west of the tin-smelting works in the valley 

 above Tolcarn, a quarry has been opened for the purpose of ex- 

 tracting the hard pyroxenic rock to be used as road material. The 

 killas is here seen between two beds of crystalline rock, and differs 

 from that in the Tolcarn quarry in containing thin foliations of 

 hornblende, with perhaps a little mica, in planes parallel with its 

 cleavage. This rock appears to be intermediate in character be- 

 tween the ordinary clay-slate of the district and the crystalline fissile 

 rocks, which now remain to be described. 



These rocks, which are generally fine-grained, vary in colour from 

 bluish green to greenish grey, and differ considerably in the facility 

 with which they may be divided into thin plates. In some cases 

 slaty cleavage is distinctly recognizable, while in others nearly all 

 trace of such a structure has become obliterated, under which cir- 

 cumstances the rock breaks with a subconchoidal fracture ; between 

 these extreme limits various degrees of fissility may be observed *. 

 When thin sections are examined under the microscope they are 

 found to consist to a large extent of hornblende, which occurs both 

 as crystalline patches and as felted acicular crystals. Sometimes 



* It is often impossible to determine to what extent the fissility of these 

 rocks is due to true slaty cleavage, and how far it may be the result of 

 bedding. 



