THE CRETACEOUS SYSTEM. 



733 



Hippurites bioculata. 

 Spherulites ventricosa. 



Hippurites, a name derived from that of the common herb hippuris, the mare's tail, the 

 stem of which the shells are supposed to resemble, and Spherulites, shells of a globular 

 form, as the title indicates. In the cretaceous system of England, as it appears in the 

 midland counties, the green sand occupies comparatively low tracts; but in Wilts it 

 constitutes a secondary range of hills, presents high insulated masses on the confines of 

 Dorset, forms the summit of the extensive table land of Blackdown in Devon, and 

 appears at the elevation of nearly a thousand feet in Leith Hill in Surrey, thus rivalling 

 in height the chalk hills of the North and South Downs. The formation is variously 

 a loose sand, and formed into sandstone by a calcareous cement. 



Gault. This is a provincial name for beds of chalky clay overlying the green sand, 

 varying in colour from a light grey to a dark blue, holding irregular balls of argil 

 laceous ironstone and iron pyrites. The craie tufeau of the continent corresponds with 

 our English gault, but no where are the strata very persistent in their 

 occurrence, or of much importance. Marine shells are abundant, gene 

 rally distinguished by their brilliant pearly lustre, of which the engraving 

 shows an example. 



Chalk. This mineral, occupying the upper part of the system, so 

 useful in the arts and in agriculture, is a carbonate of lime, composed 

 Terebratuia digona. of nearly 44 parts of carbonic acid, and 56 of lime. It seldom, how 

 ever, occurs pure, but has earthy admixtures. Some of the foreign chalks contain as 

 much as from eight to ten per cent, of carbonate of magnesia, and are characterised 

 by minute black spots, like gunpowder grains, imbedded in them. The two divisions of 

 the chalk formation, the upper and lower, differ to some extent in their character. 

 The upper is in general friable and soft, so as to be readily scratched with the nail ; 

 the lower is harder, and is sometimes used as a building-stone. The colour of the 

 upper chalk is often a clear snowy white, while that of the lower passes into a dusky 

 grey, and even a red. The upper beds contain numerous nodules of siliceous or 

 flinty matter, arranged in layers, which are either entirely absent, or not so common 

 in the lower. In general, the appearance of stratification in the chalk is not dis 

 tinct, arising from the soft yielding nature of the mass, admitting of the beds passing 

 indiscriminately into each other. The red colour of the lower part of the Yorkshire 

 chalk is clearly seen in the cliffs of the coast between Flamborough and Filey ; and 

 this hue characterises the scaglia in the neighbourhood of Genoa, considered to be a 

 mode of chalk. 



Chalk with numerous flints, that with few flints, and that without them, may be seen 

 well exposed in the range of rocks reaching from Shakespeare's Cliff, near Dover, 

 along the coast to the South Foreland, and round that point to St. Margaret's Bay. 

 The occurrence of these siliceous aggregations, so different from the surrounding mass, is 



