10 TRANSITION LIMESTONE. 



The limestone rocks here are differently com- 

 posed, but are principally of four kinds a pale 

 gray, hard and compact ; a pale cream-coloured, 

 fine grained and sonorous : these form the upper 

 stratum of stone on our down, a recent deposit, or 

 more probably a mass heaved up from its original 

 station. The whole of this mass, running nearly 

 half a mile long, is obviously of animal formation, 

 a coral rock ; a compounded body of minute cylin- 

 drical columns, the cells of the animals which con- 

 structed the material, the mouths of which are all 

 manifest by a magnifier. The stop in the progress 

 of the work is even visible; soft, stony matter 

 having arisen from some of the tubes, and be- 

 come indurated there in a convex form ; in others 

 the creatures have perished, but their forms or 

 moulds remain, though obscure, yet sufficiently 

 perfect to manifest the fact : these tubes, by expo- 

 sure to the air for any length of time, have the 

 internal or softer parts decomposed, and the stone 

 becomes cellular. A. (Plate 1, fig. 1) represents 

 an enlarged fragment of the down limestone, with 

 the mouths of the cells ; B.C.D. are the appear- 

 ance of some of them, with the forms of the insect 

 which constructed them : E. represents the stony 

 matter by which the work is continued fixed at the 

 mouth of the cell ; Fig. 2. the same when decom- 

 posed, the animal matter having quite perished in 

 some cases, and partially in others ; Fig. 3, en- 

 larged. This stone burns to a fine white lime, and 



