of a Phonolite from the " Wolf Bock " 89 



indistinct 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 

 following results : — 



Sp. Gr. = 2-54. 



I. u. 



*Water 2 • 05 per cent 2-05 



.. 56-40 



.. 22-20 



2-61 



-97 



Trace 



1-35 



Trace 



Trace 



.. 2-73 



.. 1111 



99-88 99-42 



Anyone who has made a careful examination of the Tertiary 

 phonolites, or is acquainted with Professor Zirkel's researches on 

 them, will at once recognize the identity of their mineralogical com- 

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

 thoroughly characteristic appearance of the nepheline, which is ab- 

 solutely 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 unknown, and likely to remain so, for the rock stands 

 alone in the sea, and its actual relations with others cannot be ob- 

 served. Situated between the Land's End and Scilly Islands, it is 

 in a Palaeozoic district, disturbed and penetrated in all directions by 

 granites, porphyrites, and diorites ; few, therefore, wiU 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 geological age ; a dark-coloured augitic rock, for ex- 

 ample, would be a basalt if of Tertiary age, but must be a melaphyr 

 or a;phanite, 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 descrip- 

 tions of that rock, except that the elasohte is here represented by 

 nepheline crystals which cannot be distinguished from those of true 

 phonoHtes. 



After some hesitation, I have adopted the name of 'porpTiyritic 

 johonolite 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 



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



