OF CENTRAL AND EASTERN CORNWALL. 477 



only been completely transformed into either viridite or feathery or 

 belonitic hornblende, but the felspar has also to a great extent be- 

 come decomposed into an amorphous or somewhat granular mass. 



Numerous bands of augitic and other crystalline rocks occur in 

 the strike of the schistose beds, extending eastward from Pentire 

 Point through Endellyon to Port Isaac. Some of these belong to 

 the doleritic class ; while others are vesicular and lighter in colour, 

 the vesicles being for the most part filled by a mixture of calcite 

 with viridite or chlorite, or of quartz and chlorite. The dark- 

 coloured pyroxenic bands appear to be frequently intrusive, and to 

 break through the slates in such a way as to indicate that the 

 latter must have been solidified before the intrusion of the igneous 

 rock. The vesicular rock, sometimes called " dunstone," presents, 

 on the contrary, all the characteristics of a lava which has flowed 

 over the surface contemporaneously with the deposition of the accom- 

 panying sedimentary beds *. 



" Blue Elvans." — A dark-green trap, which at Pentire Point is 

 interbedded with the vesicular lavas, presents several bold out- 

 crops ; but since it has not been opened upon by quarrying, it is 

 impossible to obtain perfectly fresh specimens by merely breaking 

 as far as possible into the mass with the hammer. Fragments- 

 thus procured are distinctly crystalline, but show by their dull 

 colour that they have, to a certain extent, suffered from weathering. 



The presence of carbonic anhydride and of a large amount of 

 combined water in this rock, as shown by the following analysis, 

 likewise indicates that it has undergone a considerable amount of 

 alteration (sp. gr. =2*88): — 



L »' 



Water { h yg r ° metric '45 



\ combined 4*71 



Silica 38-53 



Carbonic anhydride 3*21 



Phosphoric „ *37 



Sulphuric „ trace 



Alumina , 15-41 



Ferric oxide 2-66 



Ferrous oxide 13-66 



Lime 8-95 



Magnesia 8-66 



Potassa -47 



Soda 2-61 



99-69 



The felspar in this rock is not abundant ; but a certain proportion 



* The late Sir Henry T. de la Beche, whose views with regard to the geo- 

 logy of igneous rocks were far in advance of those of the majority of his 

 contemporaries, explains the occurrence of these rocks on the northern coast 

 of Cornwall in a similar way (' Geology of Cornwall, Devon, and West Somer- 

 set,' p. 88). 



