S. Allport — Structure of Phonolite. 249 



to -g-i-o- in. across, but there are perfect hexagons which do not 

 measure more than the W o-otli of an inch ; most of them are indis- 

 tinct in outline when seen by ordinary light, but become well defined 

 when examined with a half-inch objective between crossed prisms. 



Two analyses of this rock afforded Mr. J. A. Phillips the follow- 

 ing results : — 



Sp. Gr.=2-54. 



I. II. 



iWater 2-05 per cent 2-05 



Silica 56-46 „ „ 56-40 



Alumina 22-29 „ „ 22-20 



Peroxide of Iron 2-70 „ „ 2-61 



Protoxide of Iron -97 „ >, "97 



Manganese Trace Trace 



Lime 1-47 „ „ 1"35 



Magnesia Trace Trace 



Phosphoric Acid Trace Trace 



Potassa 2-81 „ „ 2-73 



Soda 11-13 „ „ 11-n 



99-88 99-42 



Any one who has made a careful examination of the Tertiary phono - 

 lites, or is acquainted with Professor Zirkel's researches on them, 

 will at once recognize the identity of their mineralogical composition 

 with the rock here described, and will be struck with the thoroughly 

 characteristic appearance of the nepheline, which is absolutely the 

 same in both. In fact, no one would hesitate to call it a phonolite, 

 if it were known to be of Tertiary age. The age, however, is un- 

 known, and likely to remain so, for the rock stands alone in the sea, 

 and its actual relations with others cannot be observed. Situated 

 between the Land's End and Scilly Islands, it is in a Palaeozoic dis- 

 trict, disturbed and penetrated in all directions by granites, porphy- 

 rites, and diorites ; few, therefore, will hesitate to place it among 

 the older series of igneous rocks. It is at present the practice among 

 many petrologists to name rocks according to their supposed geo- 

 logical age ; a dark-coloured augitic rock, for example, would be a 

 basalt if of Tertiary age, but must be a melaphyr or aphanite, if of 

 some indefinite early age. In accordance with this absurd system, 

 the rock in question would probably be called a Foyaite, if it were 

 known to be old, as it agrees well with descriptions of that rock, 

 except that the elgeolite is here represented by nepheline crystals 

 which cannot be distinguished from those of true phonolites. 



After some hesitation, I have adopted the name of porphjritic 

 pJionolite for this rock, and will take the present opportunity of sug- 

 gesting that one name only should be assigned to all igneous rocks 

 composed of the same constituent minerals, irrespectively of their 

 age ; or, in other words, that we should assimilate the nomenclature 

 to that employed in the sedimentary rocks. We speak of Carbon- 

 iferous or Tertiary sandstones, etc., why not Carboniferous or Tertiary 

 dolerites or melaphyres ? When the age cannot be precisely ascer- 

 ^ Of which -94 was lost in water-bath. 



