NOTES AND EXHIBITS. 1079 



(5) As thin linings to steam-holes formed in basalt. 



(6) As thin surface-films on stalactites in caves in basaltic lava. 



(7) As extensive lava-sheets as at Hawaii in the Sandwich 



Islands, and the Friendly Islands (V). 



In Europe basalt-glass is a comparatively rare rock, being 

 restricted in its occurrence to the first six modes described, its 

 massive development at Hawaii, and perhaps at the Friendly Isles, 

 being quite exceptional. In all the other cases, except the two 

 last, it has been formed through a local rapid cooling of the basalt 

 lava ; those portions which cooled more slowly having their base 

 more or less completely devitrified. 



At Vegetable Creek basalt-glass occurs as small ejected blocks, 

 or lapilli, of the size of walnuts, and these have been cemented 

 in places, chiefly by sphserosiderite (?), into a fine volcanic agglo- 

 merate. The lapilli have been found on the surface of the basalt 

 near the E. side of portion 70, parish of Arvid, and the agglo- 

 merate near the N.E. corner of portion 171, parish of Scone, to 

 the S. of Reynold's selection. The lapilli are of irregular shape, 

 subangular, and pitted superficially with steam-holes. The colour 

 of the weathered surfaces is dark bi'ownish-grey, and that of 

 freshly broken surfaces black, with a resinous or pitchy lustre. 

 The hardness is about 6. The fusibility is about the same as that 

 of natrolite, the fused bead being of a brownish black colour. The 

 powder of the basalt glass is not magnetic, or only very feebly so. 



Thin slices of this rock, seen under the microscope, show that it 

 is micro-porphyriticinstructure, consisting of a translucent, brown- 

 ish-yellow, glassy magma in which occur micro-porphyritic crystals 

 of olivine, and a few of augite, both enclosing crystallized mag- 

 netite, microscopic crystals of triclinic felspar and augite, and 

 spherulites, and globulitic strings traversing the magma and in 

 places following the cracks. The glass remains perfectly dark 

 under crossed nicols, with the exception of some locally devitri- 

 fied portions. The glass is traversed by a network of cracks 

 ■running irregularly, or grouped zonally round the large olivine 



