SOME OF THE ROCKS OF THE LIZARD DISTRICT. 927 



representing the " Saussurite." The external portions of this latter 

 contain much carbonate ; but the central mass, or core, consists of a 

 bluish-white compact silicate, or mixture of silicates which, omit- 

 ting small quantities, has the following composition : — 



I. " Saussurite" of the newer Gabbro. 



Dried at 1 00° C. 



Ignition (water with traces of carbonic acid) . . 4*80 



Silica 45-70 



Alumina 23-00 



Oxides of iron -50 



Lime 19-30 



Magnesia 4-75 



Alkali and loss 1*95 



100-00 



The compound has more affinity, as regards its percentage of silica, 

 with anorthite than with labradorite ; but the presence of so much 

 magnesia and its poverty in alkali must lead us to regard it as a 

 mixture rather than as a mineral species worthy to be classed with 

 the felspars *. 



II. 



The diallage associated with the above is a well-defined mineral, 

 and but slightly affected by ignition or acids. Subjoined is a rough 

 analysis : — 



Dried at 100° C. 



Ignition (water) 1-30 



Silica 50-10 



Alumina 6-77 



Oxides of iron 6-90 



Lime 17*46 



Magnesia 17*47 



100-00 



No alkali was found in the specimen examined. The amounts of 

 lime and magnesia, being so exactly equal, indicate a mineral which, 

 in its chemical aspect, is almost equally allied to the augites and 

 the hornblendes f . 



* See p. 895 for the history of this mineral. Perhaps the magnesia may be 

 accounted for by the presence of pale-coloured hornblende, which, though almost 

 microscopic, is often present in considerable quantities. — T. G. B. 



t This may explain the extreme facility with which this diallage is replaced 

 by hornblende, see p. 895. 



