File:The Quarterly journal of the Geological Society of London (12893460475).jpg

Yol. 53.. GRANITE FROM THE MIDDLE CHALK OF BETCHWORTH. 217
felspar are more or less cracked and recemented

there are some
strain-shadows in the former, and I am disposed to attribute a
cross-hatched structure, which occurs ' sporadically ' in the latter,
to the same cause. The rock, in short, is gneissoid from pressure,
and resembles those which are rather common in the same regions
as the last-named.
' I have compared these slices with one from the original Purley
boulder given to me by Prof. Judd. In this the felspar is more
distinctly idiomorphic, the quartz has slightly larger cavities, there
is hardly any mica (biotite), and one small grain, I think, of horn-
blende. One or two zircons occur, and also a few very small
Fig. 2. — Microscopic section of Boulder B.
X30.
The dark grains are garnets containing much opacite. The clear grains are
quartz. The white mica can be recognized by its cleavage-lines. The
rest of the slide is felspar. The rather wavy dusty lines are cracks, stained
or bordered with decomposition-products.
minerals, probably of secondary origin, together with one slightly
larger, which is oblong in form

these I cannot identify with
certainty, and need not discuss on the present occasion. This rock,
though less affected by pressure, might have come from the same
region as the others. I have also compared Mr. Stebbing's
specimens with a slice from a boulder obtained by Mr. Starkie
Gardner from the Kentish Gault, which is almost wholly composed
of quartz and felspar, and is much pressure-modified.'
Q. J. G. S. No. 210.