THE CORALLIAN ROCKS OF ENGLAND. 293 



not properly oolitic, but contains here and there small unrounded 

 grains ; it has been much subject to infiltration since its deposition, 

 which has taken away all the substance of the fossils, leaving 

 only empty spaces as casts. These lie throughout the mass of the 

 stone, and are abundant, including Phasianella striata, Chemnitzia 

 heddingtonensis, Cerithium muricatum, C. limceforme, Littorina mu- 

 ricata, Alaria, sp., Lucina Moreana, Lima rigida, Cidaris Jlori- 

 gemma, and RhabdopJii/llia. This, then, is a fourth form in which 

 these upper rocks appear. This is continued for some distance along 

 the strike, and may be seen again under a very slightly different 

 form near Preston, where the beds are very irregular, and blue and 

 moist from the presence of clay beneath, and have a considerable 

 amount of pisolite. Here Echinobrissus scutatus is fine and abun- 

 dant, and Nerincea fasciata and Unicardium sulcatum are added. 

 The depth of each of these quarries is about 12 feet. 



Continuing in the same direction, we find at Lyneham Hillocks, 

 and all along the narrow band that stretches towards Wootton 

 Bassett, the Coral Rag alone representing the series above the sands. 

 It is here in its normal form, with layers of Thamnastraean coral- 

 growths, and drifted Thecosmilice, with associated bands of brash, 

 rubble, and clay. Splendid sections of these reefs may be seen at 

 various points along this line of road, and many of the usual fossils 

 noted ; but though exceedingly interesting to see, they present no 

 special feature calling for remark. 



At Calne, as before stated, the rocks on the horizon just described 

 are the highest below the Kimmeridge Clay ; but towards Hilimarton 

 and for some distance further, indications of supracoralline beds very 

 similar to those that lie in the cracks and uneven surfaces of the 

 older Corallian rocks in North Dorset make their appearance. These 

 ruddy, ferruginous, sandy clays are mapped by the strong colour 

 they give to the soil, and can seldom be seen in situ. An opportu- 

 nity, however, is afforded in the above-mentioned limestone-quarry 

 at Preston. Here about 2 feet of rotten ferruginous rock is seen 

 overlying the limestone, having, towards the base, nodules and frag- 

 ments of a biplicated Ammonite, Belemnites, and a large oyster, 

 similar to one that is found in an analogous position in Yorkshire. 

 We know not how far this deposit may once have extended, and have 

 been removed or covered by the Kimmeridge Clay ; its perpetual 

 reappearance in the same character, independent of the underlying 

 beds, leads to our regarding it as a widespread indication of coming 

 change, being the first introduction, if we except the intercoralline 

 clays of Hilimarton, of the argillaceous conditions that were to 

 succeed. 



As a further illustration of the development of the Corallian beds 

 in this district, we think that two sections visible near the escarp- 

 ment, where the whole series is so thin as to have representatives of 

 each portion in a single quarry- face, may not be uninteresting. In 

 the neighbourhood of Clack the country presents a beautiful natural 

 plateau, in which the surface is composed of not much more than a 

 single bed over a considerable area. This plateau is upheld, and its 



