The Bocks of Tristan d'Acunha. 51 



apparently was powerless to act on the magnetite crystals. It shows 

 that the derived fragments have not been subjected to long exposure 

 to heat after being embedded. 



Inaccessible Island. 



Slide A fine-grained, black basaltic rock, with small crystals 



Enstatite °^ au git e (enstatite ?), showing irridescent tarnish colours 

 felspar and enstatite ; vesicles much drawn out and compressed ; 

 Andesite. sl ^gJ ° n the upper surface. 

 Under the microscope : — 

 A felted mass of laths showing a somewhat indistinct nuidal 

 structure around the long drawn-out vesicles. 



The greatest bulk of the crystals are felspar laths shown in some 

 three grades of size, the smallest ones sometimes aggregated with 

 fibrous brushes. The middle size are long, slender laths sometimes 

 with sharp terminations, but the glassy matrix often dips into the 

 termination, making it ragged; glass often also forms an axial string 

 down the lengths of the laths. The largest size attain almost por- 

 phyritic dimensions and are crowded with irregular glass inclusions. 

 The felspar laths are often clasped by the enstatite crystals. The 

 larger felspars are monoclinic, that is, they are sanidine. The 

 matrix is composed of felspar laths, enstatite, magnetite, and brown 

 glass. 



The enstatite is a light yellow, strongly refractive mineral showing 

 in rhombic sections and square-ended laths ; the crystal form is 

 unusual and is simply made of the prism and basal plane. Nearly 

 always there is a tubular hollow down the centre filled with a honey- 

 brown glass. The pinacoidal cleavage is often seen running 

 diagonally across the rhombic sections, the crystals, however, 

 usually break up with irregular cracks. Interference colours never 

 exceed yellow. The average breadth of the laths is *02 mm., 

 but some are larger ; there is a number of minute granules of the 

 same mineral in the matrix. Rarely the enstatite has adherent to 

 it a crystal of magnetite, but there is no increase in the iron content 

 of the silicate which would be shown by a higher colour and con- 

 sequent pleochroism. The macropinacoids are sometimes present ; 

 rarely the laths are grown together in a bunch. The very small 

 crystals of the matrix often show the prisms with the more usual 

 pointed terminations caused by the presence of the macrodomes. 



