32 ME. r. ETJTLEY ON TUFACEOUS EHTOLITIC [Feb. I9OI,. 



reflected liglit under low powers ifc is difficult to say more than 

 that some of them are dark or even black, while others are opaque^ 

 and white. Since, by this means of illumination, none of the former 

 display any brassy colour or lustre, they may probably be regarded as 

 magnetite, while the latter are evidently pseudomorphs of a 

 white substance, possibly replacing crystals of spinel or garnet. 



Throughout the groundmass occur a great number of apparently 

 circular, dusty-looking, brownish to brownish-green spots, averaging 

 about ^-^^inch in diameter (PI. I, fig. 1); but occasionally they are 

 elliptical, or appear to have been drawn out in the direction of 

 the rather obscure fluxion-banding. In polarized light these spots 

 undergo extinction between crossed nicols, and are seen to be 

 small fragments of crystals surrounded by a narrow isotropic border 

 of the groundmass (PI. I, fig. 6). The fragments scarcely ever 

 exhibit any definite crystal-form ; but some of them may here 

 and there be found which show what is apparently the edge 

 of a crystal, and sometimes obscure traces of cleavage parallel 

 to that edge. Selecting such examples, it is found that the 

 extinction-angle made to this edge is sometimes 0°, but very 

 frequently about 37° or more : the mean of half a dozen measure- 

 ments gave an angle of 37° to 40°. The mineral may be regarded as 

 augite. Itshowsno apx»reciablepleochroism, and in some instances 

 the fragments exhibit indications that the obscure traces of cleavage 

 intersect nearly at a right angle. The nuclei of some of the small 

 spots apparently consist of fragments of felspar, and in some cases 

 chlorite may possibly be present. It does not seem that the spots 

 are in any way due to the infilling of very small vesicles, but rather 

 that the diminutive fragments represent a shower of volcanic dust 

 incorporated with the lava. 



The porphyritic crystals and fragments of felspar are some- 

 times comparatively little changed, but the majority are partly or 

 wholly represented by alteration-products. The larger porphyritic 

 crystals are in some cases orthoclase ; in others, judging from, 

 their extinction-angles, they are oligoclase and andesine. In 

 some instances they have been partly or wholly replaced by 

 muscovite (PI. I, fig. 1). This figure also shows the spotted 

 character of the groundmass or devitrified glass. 



In other cases the crystals have been so greatly corroded that 

 they no longer exhibit any definite crystal-boundaries, but merely 

 form irregularly-shaped, spongy-looking patches which may be 

 regarded as greatly altered and highly corroded vestiges of felspar- 

 crystals. See PI. I, fig. 2, where a patch in the upper right quadrant 

 is in a position of extinction, while other similar patches are brightly 

 illuminated. Occasionally these irregular patches are connected 

 by strings of the same substance or lie in close proximity (as 

 in PI. I, fig. 3), when it is possible, from their simultaneous extinc- 

 tion, to see that they form portions of the same original crystal,, 

 of which they are now mere vestiges. 



Prom this it would appear that these crystals were derived from 

 an earlier source than the less corroded felspars in the rock. 

 AVhether their corrosion is due to fusion in the rock in which they 



