167 



In addition to killas and these green slates there is a variety of 

 slaty rock which is somewhat greyer in colour than the fissile 

 hornblendic rocks. This contains a larger amount of magnetite ; 

 and the proportion of colourless base is also more consider- 

 able. Sections prepared from a rock of this description, which has 

 been quarried near Penlee Point, on the right of the road from 

 Newlyn to Mousehole, were found to consist of a mixture of horn- 

 blende, viridite, and magnetite enclosed in a transparent base. 

 The granular colourless base is traversed by parallel undulating 

 irregular bands of feathery green crystals; spongy magnetite is also 

 present, and is either disseminated throughout the mass, or forms a 

 border to bands or patches of hornblende. The base is thickly 

 interlaced with belonitcs; and its granular character becomes appa- 

 rent on being examined by the aid of polarized light. 



A freshly broken specimen of a greenish-grey rock of this class, 



from a quarry in the village of Newlyn, was analyzed in duplicate, 



and afforded the following results (sp. gr. 2-89) : — 



I. II. 



Water I h Ig rometric ' 19 '20 



Warer \ combined -55 -63 



Silica 50-57 50-51 



Phosphoric anhydride -12 -13 



Alumina 19-65 19-69 



Ferric oxide 1-46 1-28 



„ pcrsulphide traces traces 



Ferrous oxide 7*24 7-40 



Manganous oxide trace trace 



Lime 7-47 7-38 



Magnesia 7*93 8-04 



Potassa 1-08 1-00 



Soda 3-46 3-41 



1-72 99-67 



tained only 37 "84 per cent., while the percentage found in a specimen from 

 Fredriksvarn was still less, or 37'34 per cent. 



A specimen of black crystalline hornblende from a vein in the cliff near 

 Botallack, analyzed by A. Gr. Phillips in the author's laboratory, afforded the 

 following results (sp. gr. 324) : — 



Water -94 



Silica 40-12 



Alumina 16 - 75 



Ferric oxide 432 



Ferrous oxide 1270 



Manganous oxide 3*42 



Lime 12-37 



Magnesia 6 - 82 



Potassa trace 



Soda 2-65 



100-09 

 Another dark-coloured crystal from a neighbouring locality contained 

 39 "73 per cent, of silica; it is therefore evident that the proportion of this sub- 

 stance in Cornish hornblendes is sometimes unusually low. 



