basin, containing the bones of birds and fish, but an important difference is that the English strata are destitute of gypsum.
We sometimes meet with magnesian limestone subordinate to the first flœtz or mountain limestone. Sometime ago I discovered a very beautiful sparry dolomite lying in conformable strata upon the mountain limestone near Ross in Herefordshire. This variety contains 44 per cent. of carbonate of magnesia. A small ridge of rock, about four miles north-west of Bristol, upon which Lord de Clifford's house is built, is entirely composed of a magnesian limestone abounding in shells, entrochi, and madrepores; and in an adjoining hill which overlooks Blais Castle it occurs, as far as I can understand, interstratified with the mountain limestone. A specimen of this variety I find to be composed of
| Carbonate of lime | 58 | |
| Carbonate of magnesia | 38 | |
| Oxyd of iron | 1 | . |
| Sillica and bituminous | 1 | .5 |
| Loss | 1 | .5 |
| ────── | ||
| 100 | .0 | |
| ────── | ||
I may remark that this magnesian limestone varies remarkably in specimens taken even from contiguous situations, both in colour and other external characters. It is therefore probable that these varieties would afford slight differences in their chemical ingredients.
I am entirely indebted to Mr. Bright, of Ham Green, for being able to give the last mentioned locality of this rock; for upon informing him of my present pursuit, he desired me to examine the ridge to which I have just alluded, as he conceived it to be composed of a magnesian limestone.
