1S8 LOWER CHALK. 



be necessary to offer a few remarks, upon the substance of which they are 

 principally composed. 



Chalk* is a mineral too well known to require description, and yet 

 its characters are such as could not fail to excite attention, if less fre- 

 quently presented to our notice. 



The Sussex chalk varies in colour from pure white to a bluish grey, 

 and differs considerably in its coherence and composition. 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 inconsiderable pro- 

 portion of silex and iron. 



The harder varieties of this substance were formerly in great request 

 for building, and when protected 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, wliich have stood nearly 800 years, 

 afford a remarkable instance of this kind ; the interior 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 present, chalk is 

 seldom used in architecture, except in the construction of vaults, cellars, 

 and other subterranean works. 



The Sussex chalk forms two principal divisions, viz, ; the lower or hard 

 chalk, which is destitute of flints ; and the upper chalk, which contains 

 numerous layers of siliceous nodules, and veins of flint 'i-. 



* Various conjectures have been offered respecting the probable origin of chalk, and the 

 mode of its formation. Patrin* supposed that it was the production of three different causes. 



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 f. 



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. It contains echiniles, ta'ebratulcE, ammonites, and belemnites. — Geological Transactions, 

 Vol. iii. pp. 169. 129. 



■\ In some parts of England the beds of chalk admit of a more minute division. Those 



• Diet. d'Histoire Naturelle, Tom. vi. p. 472. t Journal de Pliysique, Tom. bojc. p. 37. 



