OF CENTRAL AND EASTERN CORNWALL. 485 



of their length, while, less commonly, cleavage-lines, forming angles 

 characteristic of hornblende, are very distinct. With this mineral 

 is associated a triclinic felspar, some of the crystals of which are 

 comparatively unaltered, while others have become partially kaoli- 

 nized. This felspar is everywhere permeated by belonites of bright- 

 green hornblende, often occurring in radial groups, as well as by 

 patches of subcrystalline viridite. Altered titanic iron is moderately 

 abundant, as is also a nearly colourless transparent augitic mineral ; 

 but no apatite was observed. Fig. 6 (Plate XXTI.) represents a 

 section of rock from this locality, magnified 14 diameters. 



Scaddick Tor, situated two miles further east, is composed of a 

 rock of similar character, but considerably coarser in texture, since 

 the majority of the hornblendic crystals are at least 13 millims. in 

 length. Thin sections are found to differ from those of the fore- 

 going rock only in that the grain is coarser, while felspar is less 

 abundant and partially replaced by a transparent though some- 

 what milky substance, enclosing grains of quartz, and is without the 

 transparent augitic mineral which sometimes occurs in rocks from 

 this district. 



At Trewint, near Five Lanes, a stone was at one time extensively 

 quarried from a broad band of " blue elvan," somewhat closer 

 in structure than that constituting Scaddick Tor, but in other 

 respects closely resembling it. This quarry does not appear to 

 have been worked for some years ; but fresh specimens are never- 

 theless readily obtainable from the rock in situ : these afforded, 

 upon analysis, the following results (sp. gr. = 3*04) : — 



Water i Vg^metric *32 



( combined • / 1 



Silica 47-72 



Phosphoric anhydride trace 



Alumina 17*72 



Ferric oxide *60 



Ferrous oxide 10*71 



Lime 10-90 



Magnesia 7'89 



Potassa -33 



Soda 3-00 



99-90 



Sections of this rock are found, under the microscope, to contain 

 no felspar, its place having been apparently taken by a small quan- 

 tity of a granular ground-mass, which occupies the spaces between 

 bacillar crystals and microlitic aggregations of hornblende ; it also 

 contains viridite, some unaltered magnetite, and numerous angular 

 grains of quartz, which do not enclose liquid- cavities. 



In its chemical composition this rock is essentially identical with 

 the ancient doleritcs from the neighbourhood of Penzance, thus 



