VI. Oviform Limestone (Oolithe).
I have observed two beds or strata of this rock: one is in the quarry of Wind Spit in the Isle of Purbeck. This quarry is open in two places. To the east, where the rock has been excavated on a very large scale, there is nothing but the coarse shelly limestone before mentioned, which as it extends to the western quarry, passes into oviform limestone. The grain of the rock quarried at this latter place, is more close, the colour uniformly white, the texture less coherent; it still however retains some fragments of shells, but they are small and broken. I found the specific gravity of one specimen 2,539.
The quarry to the north-west of the Isle of Portland, which is the most extensive, is composed almost entirely of the oviform limestone, containing but very few shells; the texture of the work is granular, the nodules very small, and united by a calcareous cement. It crumbles to pieces much more readily than the coarse shelly limestone which lies on the opposite coast of the island. The specific gravity of one specimen I found 2,464; thus giving for the mean specific gravity of the two specimens here mentioned 2,5015.
“ Les oolithes,” says M. Brongniart, “ se trouvent en bancs ou en masses considerables au pied des collines ou des montagnes, dans le passage des terrains de crystallisation, aux terrains de sédiment : elles sont rares dans la chaux carbonatée compacte : on ne les a jamais vues dans la craie proprement dite; il paroit qu`elles sont particuliéres à la chaux carbonate grossiére.”[1]
- ↑ Traité élement. do Mineral. tome i. p. 203.
