OF CENTRAL AND EASTERN CORNWALL. 481 



Sections of this rock were found to consist of a transparent base, 

 enclosing numerous angular fragments of quartz and grains of 

 unaltered magnetite, together with occasional specks of iron py- 

 rites; throughout this mixture are thickly disseminated groups 

 of needles of green hornblende, arranged in nearly parallel bands. 

 This very closely resembles some of the slaty hornblendic rocks of 

 the St.-Ives and Cape-Cornwall districts, and is overlain by a stratum 

 presenting the appearance of an imperfectly consolidated ash-bed. 



Pig. 1 (Plate XX.) represents a section of this rock magnified 

 22 diameters. 



In a field near Hendra Chapel, on the left of the Launceston road, 

 three and a half miles east of Camelford, a quarry is worked upon 

 a rock of a sage-green colour, enclosing lamellar patches of a mineral 

 possessing a lustre somewhat approaching that of bastite. 



A freshly broken specimen from this locality afforded the follow- 

 ing results (sp. gr. = 2*94) : — 



Water l h ^ grometric * 24 



Water \ combined 2-64 



Silica 44-69 



Alumina 17*58 



Ferric oxide 4-52 



Ferrous oxide 7*10 



Manganous oxide trace 



Lime 10-54 



Magnesia 9*81 



Potassa trace 



Soda 2-87 



99-99 



When thin sections are examined under the microscope, the 

 lamellar striated mineral before referred to is found to be distinctly 

 dichroic, and to present other characteristics of hornblende. A 

 triclinic felspar is also present, as well as a considerable amount of an 

 imperfectly crystallized colourless transparent mineral, which I was 

 unable to identify, but which Prof essor Zirkel believes to be augite*. 

 With these are associated viridite, numerous crystalline patches of 

 altered ilmenite, and a few granules of quartz. 



Near the church of St. Clether a high bluff runs for some 

 distance parallel with the valley, and is composed of a slightly 

 modified dolerite, in which all the constituents frequently occur in 



* Professor Zirkel, who has kindly examined a thin section of this rock, is 

 of opinion that the nearly colourless imperfectly crystallized mineral does not 

 exhibit sufficiently characteristic peculiarities to admit of its absolute iden- 

 tification ; but he believes it to be probably a variety of augite, especially as 

 pale sahlite is of frequent occurrence in crystalline hornblendic rocks, as well 

 as in hornblendic and chloritic slates. He further remarks that its cleavage 

 and optical properties are not contradictory of this hypothesis. 



2k 2 



