478 T. W. E. DAVID, W. F. SMEETH, AND J. A. SCHOFIELD. 
MgO = 7:33 
BO ee 7 8S 
Na,O. = , 2-14 
H,O = 2:18 
100-06 
The percentage of Al,O, is high and implies the presence of 
felspar, either actual or potential. As there is little glass present 
it is probable that the augite, which is by far the most prevalent 
constituent, is itself highly aluminous. 
The porphyritic augites are frequently broken but not much 
corroded. All exhibit shelly zoning, there being generally two 
or three thin shells round each crystal. In some cases where the 
crystal has been fractured these shells continue round the fractured 
edge, showing that they have formed subsequently to the breaking 
of the original kernel. This kernel is frequently twinned and is 
greenish-yellow in colour. The anglec:¢ = 45°; this increases 
in other outer shells up to 55° in the outermost. Zoning is 
apparent in ordinary light, owing to the outer shell being of 4 
decided yellow colour, but whether this is due to change in com- 
position or merely to staining is not clear, though the former is 
probable, from the sharpness and uniformity of the coloured shell. 
One crystal differs from the majority in having a well marked 
shell with a greater angle of extinction than the kernel and out 
side this a thin shell having the same extinction as the kernel. 
The olivine is all porphyritic, idiomorphic outlines being ¢om 
mon. All the crystals exhibit a curious structure. They at? 
traversed by a dense rectangular network composed of strings of 
magnetite grains which here and there show octahedral faces but 
are less regular than skeleton crystals. (Plate 13, fig. 4.) 
The main axes of these strings are apparently parallel to the 
axes, a, and 6, the shorter offsets being parallel to the vertical 
axis. The olivine matrix is perfectly colourless and fresh, show 
ing no decomposition and none of the usual irregular cracks. In 
