171 



composed of a crystalline rock of the same class as that near St. 

 Peter's vicarage. Penzance. This, however, has experienced a 

 larger amount of alteration than the majority of those on the shores 

 of Mount's Bay, since none of the sections examined retain any 

 trace of unaltered augite or diallage, the magnetite has been par- 

 tially replaced, and apatite sometimes appears to be the only un- 

 altered mineral present. Diallage has been found in sinking a shaft 

 at Carnellow Consols, near the point of junction of the crystalline 

 rock with slates. The nature and distribution of the various minerals 

 seen in thin sections of the more highly metamorphosed portions of 

 this rock tend to the supposition that the darker beds forming the 

 base of Gurnard's Head have had a similar origin ; but if this be the 

 case, they have been subjected to still more extensive alteration. 



At Porthglaze Cove there is a hardened killas, sections of which 

 show it to be composed of a transparent quartzose base, coloured by 

 flocculent green and brown microlites, which are plentifully distri- 

 buted throughout. 



A little north of this point the cliff becomes darker, more highly 

 crystalline, and less distinctly slaty : hand specimens resemble those 

 of the rock forming the eastern base of Gurnard's Head ; but when 

 thin sections are examined a marked difference is observed. This 

 rock is chiefly composed of closely matted hornblende, among which 

 are distributed a flocculent mineral of a greyish-green colour, 

 numerous grains of magnetite, a few patches of a granular base, 

 and some grains of quartz. 



The northern side of the promontory forming the southern 

 boundary of Pen dower Cove is composed of a dark -green crystalline 

 rock approaching to black. In hand specimens it resembles the 

 bottom rock of Gurnard's Head ; an examination of thin sections, 

 however, shows that it differs materially from it, being principally 

 composed of matted green hornblende of various shades, a small 

 quantity of transparent base, and occasional grains of quartz, but no 

 magnetite. 



An analysis of this rock, made in duplicate, afforded the following 

 results (sp. gr. = 3-35): — 



I. II. 



Wit er I h Yg rometric '34 '40 



w ater \ combined 1-05 1-07 



Silica 37-72 37-63 



Phosphoric anhydride -41 -46 



Alumina 17-19 17*15 



Ferric oxide 7'75 7*53 



„ persulphide traces traces 



Ferrous oxide 15-81 16-03 



Manganous oxide -43 -40 



Lime 14-00 13-95 



Magnesia 3-41 3-52 



Potassa 1-02 -94 



Soda 1-06 -95 



100-19 100-05 



