Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/36

20 the fern '; and also ', and other obscure plant-remains, which have been regarded as fucoids. These associated organisms indicate estuarine conditions.

2. Compact limestone, which is earthy or crystalline, brown, grey, mottled or ribboned, and slaty in the bottom beds. The crystalline is chiefly carbonate of lime, but the earthy sometimes contains as much as 44.9 per cent, of carbonate of magnesia. It has a thickness of from 150 feet to 200 feet, but contains few fossils, only eighteen species having been found in it: it appears, however, to have been throughout a pelagic deposit.

3. Fossiliferous limestone, which has a thickness of about 150 feet, is generally light yellow, compact or crystalline, with little appearance of stratification. It is so rich in fossils that about ninety species have been found in it, indicating a pelagic deposit.

4. Concretionary and pseudo-brecciated limestones, are about 150 feet thick, mostly of a light yellow colour, and containing corals and mollusks, indicating generally pelagic conditions; but in some laminated or slaty beds fishes have been found in considerable numbers, of the genera Palaeomscus and Acrolepis, accompanied with a calamite, and with other plants supposed to be fucoids.

5. Crystalline, compact, and oolitic limestone, which is often laminated, and sometimes ripple-marked. Corals and Erachiopods are absent, but it contains species of Myalina and Schizodus, which point to shore or shallow water. It is about 100 feet in thickness.

The organisms of the Magnesian Limestone bring it into close relationship with the Carboniferous system, of which it properly ranks as a group or formation. Sedgwick, in 1828, pointed out this alliance, and further researches have confirmed his view: Mr. Kirkby has shown that fifteen species of animals were common to the Mountain Limestone and the Magnesian Limestone