S. AUport — Structure of PhonoUte. 247 



Fig. 6. Bundle of fibres from fractured lobe of column of recent Pentacrinus. The 

 cross-lines show the intervening membranes of the column. 



Fig. 7. Side arm of recent Pentacrinus, decalcified by yery dilute Muriatic acid, 

 showing tufts of fibres on each side of the median ridge. 



Fig. 8. Specimen of Chert with "screw-stone." In this specimen the silicified 

 membranes at one end are perfect, and extend to the circumference, but 

 more than half are broken off. Drawn of the natural size. 



Fig. 9. Magnified drawing, showing silicified margins of plates, forming part of body 

 of Rhodocrinus, apparently the pseudomorph of the membrane in which each 

 plate was enveloped. 



11. — On the Microscopical Stkuctuee and Composition of a 

 Phonolite from the "Wolf Eock." 



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 of the Geological Magazine. 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 embedded ; 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 embedded 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 brilliant coloured 

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

 larger crystals are set. In thicker sections the felspar and nepheline 

 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 feet, as it clearly indicates that both 

 the nepheline and the smaller crystals had 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 incloses 



