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VII. — On the Microscopical Structure and Composition of a 

 Phonolite from the " Wolf Bock" By S. Allport, F.G.S. 

 With a Chemical Analysis by Mr. J. A. Phillips. 



The rock described in the following paper is, I believe, new to 

 British petrology ; and as the value of microscopical analysis is not 

 yet fully recognized, a detailed description may be acceptable to 

 many readers. The specimens examined were kindly sent to me 

 by Mr. J. A. Phillips, together with a chemical analysis, which, 

 with his permission, is also added. 



The " Wolf" is a rugged rock lying about nine miles south-east 

 of the Land's End, and covered by the sea at high water. At low 

 water of spring tides its length is about 175 feet, and its breadth 

 150 feet. Its highest point at low water is 17 feet above the level 

 of the sea, whilst at high water it is covered by it to a depth of 

 2 feet. 



Examined by the eye or simple lens, the rock is seen to consist 

 of a yellowish-grey compact base, in which crystals of clear glassy 

 felspar are imbedded ; they exhibit no striae ; their fracture is sharp 

 and splintery. 



A thin section examined in polarized light with crossed prisms 

 exhibits a beautiful group of crystals of felspar and nepheline por- 

 phyritically imbedded in a fine-grained matrix composed of minute 

 crystals of nepheline, felspar, and hornblende ; when cut very thin, 

 the hornblende alone exhibits colours, the hexagonal sections of 

 nepheline being black, the rectangular white ; the felspar is also 

 either dark or light, and the general appearance is that of a mosaic 

 of dark and light stones interspersed with small brilhant-coloured 

 crystals of hornblende ; the whole forming a matrix in which the 

 larger crystals are set. In thicker sections the felspar and nephe- 

 line display fine colours, but the minute structure is not so well 

 seen. 



The microscopic constituents are for the most part evenly dis- 

 tributed throughout the base, but not unfrequently they are crowded 

 together along the sides of the larger crystals and irregular grains of 

 nepheline. This is an important fact, as it clearly indicates thaf^ both 

 the nepheline and the smaller crystals h id been formed while the 

 surrounding mass was still in a plastic state ; it would also appear 

 that the felspar was the last to crystallize, as it frequently encloses 

 crystals ot nepheline and hornblende, which must have been caught 

 up in it at the time of consolidation. 



The nepheline occurs in the sections in hexagonal and rectangular 

 forms, or as imperfect crystals and hregular grains ; some are per- 

 fectly clear and transparent, but the greater number appear to be 

 filled with a fine grey dust ; it is sometimes equally distribute J, but 



