Prof. T. G. Bonney—Pebbles in the Cambrian at St. Davids. 317 
from a deposit of secondary quartz which is not in optical con- 
tinuity with that of the original grain. The mica flakes, practically 
colourless, vary in length up to about :006”, and often lie between 
this and -004”, they tend to lie parallel one to another, and exhibit 
bright tints with crossed Nicols. The slide contains two or three 
grains of brown tourmaline, some microlites of zircon and rutile 
(probably), some tiny flakes of iron-glance, and some opacite or 
ferrite. 
Tt seems reasonable to suppose that all these rocks were originally 
clastic, consisting of moderately worn fragments of quartz, which 
must have been derived from some more ancient and probably 
granitoid rock, with a variable amount (in one case extremely little) 
of mud, which has subsequently been converted into white mica and 
secondary quartz. 
My collection contains a fair number of slides representing 
quartzites of Paleozoic or presumably Paleozoic age, from various 
localities in Britain! and from a few other quarters. These, however, 
differ slightly from the first-named specimen, and considerably from 
the others: the alteration of the constituents, especially in regard to 
the more earthy, being less complete. But the second specimen 
bears in some respects a close resemblance to certain quartzites 
which I obtained from the Huronian Series near Sudbury in Canada.? 
In these we find mica flakes, if not developed, at any rate completed 
in sitt. True, in most of these specimens the mica is brown, but 
white mica may also be found, especially in one whitish quartzite 
which much resembles this rock from St. Davids. Again, the third 
fragment from that locality is different from any specimen of 
Palzozoic quartzite which I have ever examined, and closely 
resembles the schistose quartzites or quartz-schists which I have 
found associated with the more fine-grained mica-schists and chlorite- 
schists in the Alps and elsewhere. It almost exactly resembles one 
described by me® from above Windisch-Matrei (Tyrol), except that the 
last-named is slightly coarser in texture and a little more definitely 
foliated. Both the second and third specimens from St. Davids 
present some resemblance to a quartzite* which, at Pen-y-Pare, near 
Beaumaris (Anglesey) occurs in association with one of the greenish- 
grey micaceous schists, which are so abundant in that island. 
The quartz grains in the more highly-altered quartzose rocks of 
clastic origin exhibit under the microscope (as no doubt has often 
been remarked) a fairly distinct peculiarity of aspect. The edges 
of the individual grains appear, as it were, fused together. The 
line of junction is often slightly wavy, irregular, almost inter- 
1 eg, N.W. Scotland, Hartshill, Lickey, Wrekin, Stiper Stones, N. Wales, 
Cherbourg, the Ardennes, pebbles from Carboniferous and Triassic rocks, ete. 
2 Described in Q.J.G.S. vol. xliv. p. 32. 
3 Q.J.G.S. vol. xlv. pp. 87-106. 
4 See Q.J.G.S. vol. xxxix. p. 47 (note). Prof. Blake (éd. vol. xliv. 475, 508) 
calls this a quartz knob, and seeks to show that it has more resemblance to a vein 
product than to a rock of clastic origin. I cannot, of course, answer for the 
specimen which he examined, but can only say that if my slide does not represent 
a quartzite, I have never seen one. 
