482 



an unaltered state. This rock is associated with various bands of 

 slate or shale, which are probably consolidated doleritic ash-beds. 



" Dunstones." — By this name the ancient lavas of Northern 

 Cornwall are locally distinguished. These, when freshly broken, 

 are greenish-grey in colour, and have a finely amygdaloidal struc- 

 ture. They sometimes occur in amorphous masses of considerable 

 depth, divided into blocks by joints crossing one another at various 

 angles ; but in other cases they form distinctly foliated sheets of 

 varying thickness. 



The latter variety of conformation is apparently often due to a 

 former state of fluidity, while in other instances the rocks exhibiting 

 this peculiarity may have been deposited in the form of ash. The slaty 

 structure of such rocks may also, in some cases, have been produced by 

 the operation of influences by which the lamination of the associated 

 sedimentary beds has been effected. By weathering, ferrous oxide 

 becomes peroxidized and hydrated, imparting a dark-brown colour to 

 the mass, which becomes spongy from the removal of the calcite, 

 with which a large proportion of the vesicles are filled. 



Two specimens of lava from this district afforded the following 

 results by analysis : — 



I. II. 



vv ater j combined 3 . 98 4 . 30 



Silica 43-23 40-05 



Carbonic anhydride 2-61 3-67 



Phosphoric „ -97 *63 



Alumina 21-37 20-46 



Ferric oxide 1-69 1-83 



„ persulphide -33 trace 



Ferrous oxide -. 9-53 12-66 



Manganous oxide . . trace trace 



Lime 6-66 6*62 



Magnesia 3-57 4-28 



Potassa trace trace 



Soda 5-63 4-82 



100-08 99-88 

 Specific gravity 2-82 2-83 



These lavas, from their greater porosity, have undergone a larger 

 amount of alteration than the more compact doleritic rocks ; and it 

 consequently becomes impossible to institute a strict comparison 

 between their original compositions. If, however, the lime be 

 regarded as having formed an integral part of the unaltered rock, 

 and allowance be made for the presence of carbonic anhydride as 

 well as for the increased proportion of combined water present in 

 these lavas, their chemical composition will not be found to differ 



