Outlines of Geology. 27 



Echini are very characteristic of chalk, and among them many 

 species, and one genus, are peculiar to it. It also includes star 

 fish, encrini, many madrepores, alcyonia, and sponges. 



The forms of sponges and of the alcyonia and echini, are not 

 uncommon in the flints ; also casts of ammonites, and of nautili. 

 These remains are not equally abundant in all parts of the chalk 

 strata, nor are the strata themselves in all places similar. The 

 upper beds of chalk abound in flints, which are usually disposed 

 in regular horizontal layers, though there are cases in which, 

 from some derangement that appears to have occurred to the 

 strata, the flints are nearly vertically arranged, as in the Isle of 

 Wight, and on the Dorsetshire coast. But, not the least remark- 

 able fact, in respect to the perpendicular flints is, that they are 

 generally splintery and broken, while those which are horizontal 

 are in the usual state of rounded nodules. As the opposite coast 

 of Dorsetshire exhibits a very similar arrangement, it is ex- 

 tremely probable that this chalk ridge has once been con- 

 tinuous, and that the shock or catastrophe that has broken 

 it down, has also caused those inclinations, and that verti- 

 cally of the strata once horizontal, that I have just adverted 

 to. In the lower chalk strata, the flints become less abun- 

 dant, and it frequently has a gray colour, and is argillaceous. 

 These strata may be seen near Ryde in the Isle of Wight, and 

 at Guildford and Dorking in Surrey, and the lime which such 

 chalk affords derives certain peculiarities, as far as its use in 

 making mortar is concerned, hi consequence of its aluminous 

 character. The greater part of the chalk hills in Cambridgeshire 

 are also composed of the lower or gray chalk, and they gradually 

 pass into a kind of gray clay called gault, and into several 

 varieties of argillaceous loam. In the neighbourhood of Deal, 

 and in some parts of the Isle of Wight, the flint assumes the 

 appearance of flattened or tabular masses, which are sometimes 

 lost in a very thin edge. Near Freshwater in the Isle of Wight, 

 these tabular flints are arranged in diagonal layers, crossing 

 each other in opposite directions. 

 v Although the flinty chalk is not always found upon that without 



