OF THE SOUin OF THE ISLE OF MAN. 447 



The porphyritic constituents are large, fairly fresh crystals of 

 olivine. These arc idiomorphic, and show the brachypinacoid (010), 

 combined with the brachydonie (021). An average-sized crystal 

 measured 'lli'^ millim. long from the acute angle between the 

 brachydomes (021) to the similar opposite angle. A large specimen 

 was twice as long. The angles in some specimens are rounded, in 

 others sharp. The crystals are traversed by irregular cracks, along 

 "wliich alteration has taken place, the centre of the cracks being 

 occupied by black iron-ore (magnetite?), with green serpentine on each 

 side of it. In most cases not more than a quarter of the area of each 

 crystal has been thus altered, the spaces between the cracks being 

 almost unaltered. In many of the olivine-cr5'stals are inclusions of 

 chromite or picotite. In several instances the olivine crystals are 

 corroded by the groundmass, which occupies gulf-like spaces in 

 them. These gulfs are often partly occupied by porphyritic plagio- 

 clase crystals ; indeed the porphyritic plagioclase is most abundant 

 around the corroded olivines, and seems to have been formed in 

 connexion with their corrosion, in these cases the lath-shaped 

 crystals being often in close contact with the olivine, and having 

 their length parallel with its margin. 



The porphyritic plagioclase occurs in lath-shaped sections ; an 

 average specimen measured "625 millim. long x '050 broad, a large 

 one -915 millim. long x '105 broad. Although in most cases the 

 porphyritic plagioclase is well marked off from that of the ground- 

 mass, yet crystals occur of which it is difficult to say whether they 

 belong to the intratelluric period or to the groundmass. 



The groundmass consists largely of small lath-shaped plagioclase 

 crystals or microlites. A lath of average size measured '09 millim. 

 long X '006 millim. broad. The space between the felspar microlites 

 is filled with apparently ophitic augite and grains of magnetite. It 

 must be noted that the groundmass forms the great bulk of the rock, 

 the porphyritic constituents occurring in it as isolated crystals or 

 aggregates. It varies considerably in the same section. In some 

 parts felspar microlites are comparatively scarce, and a very finely 

 granular, greyish, faintly double-refracting groundmass is present. 



Glassy base not made out with certainty. The structure of the 

 groundmass appears to be pilotaxitic*. 



YI. SuPPLEMEITTARr OBSERVATIONS. 



Section 250. — About 170 feet S. 63° E. of the western end of the 

 causeway connecting St. ^[ichael's Island (Fort Island) with Lang- 

 ness, there occurs in the slates an igneous rock differing considerably 

 in appearance from that at Gullet liuigh. It is exposed both below 

 high-water mark and in the field above the reach of the tide. 



The rock is medium grained, mottled dark green and white, and 

 weathers brown. The dark green colour is due to hornblende, the 

 crystals of which can be readily made out, embedded in a matrix of 

 white or greenish altered felspar. Sp. grav. 2-95. 



* Kosenbusch, ' Mikr. Physiogr. d. massig. Gest.' (1887) pp. 466, 727. 



