CHALK FORMATION. 
71 
description, yet its characters are such as could not 
fail to excite attention, if less frequently presented 
to our notice. 
The Sussex chalk varies in colour from pure 
white to a bluish grey, and differs considerably in 
its coherence and comjiosition. It has an earthy 
fracture, is meagre to the touch, and adheres to 
the tongue ; it is dull, opaque, soft, and light, its 
specific gravity being about 2*3. It is composed 
of lime and carbonic acid, and contains an incon- 
siderable proportion of silex and iron. 
The harder varieties of tliis substance were for- 
merly in great request for building, and, when 
j)i’otected from the influence of the atmosphere by 
a thin casing of limestone or flint, proved very 
durable. The ruins of the priory of St. Pancras, 
near Lewes, which have stood nearly 800 years, 
afford a remarkable instance of this kind ; the in- 
terior of many of the walls are six feet thick, and 
are entirely formed of chalk, the outside having a 
facing of Caen-stone and squared flints. At pre- 
1. Animal earth, proceeding from the decomposition of organic 
bodies. 
2. Calcareous lava ejected by submarine volcanoes. 
3. Detritus of calcareous mountains. 
Delametherie imagined it to have been deposited by water in a state 
of great agitation.* 
In Ireland, the chalk acquires a degree of hardness equal to that of 
compact limestone. In its geological position, and in the nature of its 
fossils, it corresponds, however, with that of England, with which it is 
considered to be entirely identified. In many places it is covered by 
basalt, and its hardness is probably attributable to the high temper- 
ature to which it has been subjected from this cause. It contains 
ec/iinites, terebratul<e, ammonites, and belcmnit^s. — Geological Transactions, 
vol. iii. pp. 129. 169. 
* Journal de Physique, tom.Mxxx. p. 37. 
F I 
