74 
UPPER CHALK. 
layers of siliceous nodules into beds that vary from 
a few inches, to several feet in thickness, and 
wliich, in some localities, are traversed by ob- 
liquely vertical veins of tabular flint, that may be 
traced for many yards without interruption. These 
are sometimes disposed horizontally, and form 
a continuous layer of thin flint, of considerable 
extent. 
The nodular masses of flint are very irregular in 
form, and variable in magnitude : some of them 
scarcely exceeding the size of a bullet, while others 
are several feet in circumference. Although thickly 
distributed in horizontal beds or layers, they are 
never in contact with each other, but every nodule 
is completely surrounded by the chalk. Their ex- 
ternal surface is composed of a white opaque crust, 
consisting of an intermixture of chalk and silex, 
probably formed by a combination of the outer 
surface of the nodule with its investing matrix, 
while the former was in a soft state. Internally 
they are of various shades of grey, inclining to 
black, and often contain cavities lined with chal- 
cedony and crystallised quartz. 
When first extracted from the quarry, flint is 
brittle, has a conchoidal fracture, and feeble lustre ; 
thin fragments are translucent. Its specific gravity 
is 2*594. According to the analysis of Klaproth, 
it consists of 
Silex - 98 Oxide of iron 0*25 
Lime - 0*5 Water - 1* 
Alumine - 0*25 
It is infusible, but upon being submitted to a 
great heat, becomes white and opaque. By ex- 
