316 Prof. T. G. Bonney—Pebbles in the Cambrian at St. Davids. 
subangular to rather rounded in form, which are often about -015” 
to 02” in diameter. It is difficult to be quite certain of the boundaries 
of the original fragments, but secondary quartz more or less in 
optical continuity with the original grain has undoubtedly been de- 
posited to some extent. Here and there a tiny flake of light-coloured 
mica, or a little opacite or ferrite, may be seen, indicating no doubt 
the residue of the dusty material once present in the original sand- 
stone. One or two of the fragments exhibit a compound structure. 
Very minute fluid cavities with bubbles are rather numerous in the 
quartz grains; but on these, and a few tiny accidental microliths, it 
seems needless to dwell. 
The next specimen comes from a block of nearly the same size. 
It is a fine-grained quartzite, slightly rougher in texture and fracture 
than the last, and of a purple-grey colour. Examination with a 
lens shows that, though composed mainly of quartz, a good many 
tiny flakes of silvery mica are present. Microscopic examination 
shows that the quartz grains, though rather smaller in size, are very 
commonly about 012” diameter, and somewhat rounded or bluntly 
polygonal, but slightly wavy in outline. Occasionally they are 
separated by a flake of mica: more often a few specks of opacite or 
ferrite indicate the position of the original contact surface. Doubt- 
less the original grains have been augmented by deposit of secondary 
quartz; but, even more than in the former case, it is difficult to fix 
the position of the old surface. Still I should infer that the grains 
had once been fairly regular in outline. Fluid cavities appear to be 
less numerous than in the former case. The mica is white or a very 
pale greenish tint; it occurs sometimes in isolated flakes, but is fre- 
quently associated and occasionally almost forms “nests,” when it is 
usually mixed up, more or less, with dark granules, which are prob- 
ably, in part at least, hematite and magnetite. The larger flakes 
are about ‘01” in length, but the majority are smaller. The sections 
are fairly rectilinear in outline: the shorter ends not appearing either 
ragged or “nipped.” Even if originally fragmental, these flakes 
must have been subsequently enlarged, and I suspect that they have 
been to a great extent, if not wholly, developed in siti. Possibly 
some tiny crystals of zircon and rutile are present, with perhaps a 
grain of epidote; but these accessories are so unimportant, that I 
have not cared to waste time in attempting to determine them with 
precision. 
The next fragment is from a fairly rounded stone about four 
inches long. This quartzite is whitish in colour, and in structure 
is fine-grained and rather schistose. Microscopic examination 
shows that quartz is the principal mineral. The grains commonly 
present ‘ragged’ outlines, and are unequal in dimensions, tending 
to lie with their longer axes parallel. When viewed with crossing 
Nicols each grain exhibits a border tinted rather differently from 
the inner parts. This might be produced by the overlapping of 
the irregular edges of two contiguous grains; but as the tint in 
some cases continues practically uniform all round the grains, it 
must result either from a fairly regular thinning of the latter, or 
