At Potton and Gamlingay in Bedfordshire, and in the neighbourhood, this green sand is highly ferruginous, and the roads and fields present that peculiarly dark-red colour which is first singular and then wearisome to the eye. In the case of the Potton beds, the red colour is caused by oxidization or rust of iron; in the neighbourhood of Cambridge, &c., where there is the _green_ sand, this is owing to the influence of “chloritous silicate of iron.” Then we have the galt or gault, a local term of which we cannot trace the etymology. The gault, however, is not of great thickness, but is likely to be the most interesting department of the Chalk to the beginner, on account of the abundance and peculiar appearance of its fossils. A ramble under the cliffs at Folkstone,[106] where the gault may be seen in perfection, will amply repay any one for toil, dirt, and a few slips and bruises. He will there find evidences of a prolific and prodigal bestowment of life in the innumerable fragments of organic remains every where observable; and if he be patient,—if he won’t go running on from spot to spot, saying, as some do, “Oh, there’s nothing here;” if he will just persevere in a minute examination of every spot where organic remains may be detected, he will not come away without his reward in ammonites, hamites, and other cephalopodous mollusks, and most of them with that peculiar nacreous or mother-of-pearl lustre upon them which renders the fossils of this period so beautiful and attractive. Only we caution the explorer not to buy of the so-called guides. At Dover and Folkstone the rogues have a knack of getting a lump of gault, and sticking into it one or two common pyrites, which are very abundant in the cliff, bits of shell, ammonites, &c.; they then offer this conglomerate for sale, all rounded and smooth, assuring you upon their “sacred honour,” the honour of men who always draw upon their imagination for their facts, that they would not ask so much for it, only on account of its excessive rarity. As good economists always avoid cheap houses, and go to the best shops, so let the young geologist always go to the best shop: let him go to the cliff with his hammer, and work for himself. We picture a few fossils from the gault, only regretting that it is out of the power of our artist to convey their lustrous colours, as well as their curious forms. [Illustration: 1. NATICA CANICULATA. 2.} VENTICRULITES. 3.} 4.} ROSTELLARIA MARGINATA. 5.} 6.} 7.} CATILLUS CRISPI. 8.} ] [Illustration: Fossil] Then comes, lastly, the _Chalk_: that is, the _white_ chalk, divided into lower and upper; the _lower_ being harder and mostly without flints, and the upper characterised by layers and bands of flint, sometimes nodular, as in Cambridgeshire, and sometimes flat almost as a pancake, as in the neighbourhood of Woolwich.