Aug., 1889. 
PETROLOGY OF LOCAL PEBBLES. 
177 
Another curious thing may also be observed in the same 
pit, namely, hollows in the sand, which, on careful examina¬ 
tion, are found to have the usual ovoid shape of the pebbles of 
the bed, but to contain onlv a few internal casts of shells or 
encrinite stalks lying loose in the hollow, showing that the 
cavity was originally occupied by a pebble of limestone, 
which has been dissolved out by the carbonic acid contained 
in the water which has percolated through the mass since its 
deposition. I should think also that it is evidence that the 
crush which has cracked the pebbles took place at a com¬ 
paratively early epoch of their existence, as we might expect 
an empty space to be disturbed by such a pressure. In a 
cutting on the Midland Railway to Walsall, a short distance 
to the west of this place, I observed, during the construction 
of the line, a multitude of small faults (with throws of from an 
inch or so to a foot) extending over about half a mile. Every 
now and then one could be met with of more considerable 
throw, say two or three feet, and the pebbles were all 
perfectly crumbly under the hammer, seeming perfect out¬ 
wardly, but simply falling to pieces at the slightest tap; a 
circumstance which had been of great annoyance to the 
contractors for the line, as they had looked to this stretch of 
gravel for ballasting purposes. 
Of the quartzites we recognise several varieties. One is 
pure, dense, white, with a fracture crystalline and glistening 
almost like loaf-sugar; another is almost black, another 
honey-yellow, another banded. We find occasional examples 
also of a less solid-looking rock, more like that of the Lickey 
and Hartshill, and, among the quartz grains of this, bright 
pale green grains are observable. They are of a soft, earthy 
consistence, and of course totally disappear during the 
preparation of thin sections. 
A somewhat closer observation of the principal varieties 
of quartzite, as they show themselves under the microscope, 
opens up several points of interest, and I will describe the 
main peculiarities of eight sections which I have made. 
(1) A dark quartzite (1) from Sutton. 
The component quartz grains are mostly angular and of 
moderate size, averaging *005 to *003 of an inch in diameter. 
They are cemented by a quartz which is in most parts cloudy 
and apparently dirty. Some of the original grains are also 
cloudy, with minute inclusions, while others are very pure and 
clear. The irregular clouding between crossed nicols due to 
strain by pressure is very common. In the quartz grains are 
contained many minute rounded granules with high double 
refraction, showing therefore in brilliant colours between 
