Fig. 86. Fig. 87.
Fig. 86. The Dirt-bed of the Island of Portland, containing stumps of fossil Cycadaceæ in an erect position.
Fig. 87. Cycadoidea megalophylla (Mantellia nidiformis of Brongniart), a subglobose depressed trunk, with a concave apex, and with the remains of the petioles disposed in a spiral manner, the markings being transversely elliptical. It is found in the Oolite of the Island of Portland, in a silicified state.
There is an absence of true coal-fields in the secondary formations generally; but in some of the Oolitic series, as in the lower Oolite at Brora, in Sutherlandshire, and in the north-east of Yorkshire, and the Kimmeridge clay of the upper Oolite, near Weymouth, there are considerable deposits of carbonaceous matter, sometimes forming seams of coal which have been worked for economic purposes.[19] Some suppose that the Brora coal was formed chiefly by Equisetum columnare (Fig. 82). In the sandstones and shales of the Oolitic series, especially in the lower Oolite of the north of England, as at Whitby and Scarborough, as well as in Stonesfield slate, the Portland Crag of the middle, and the Portland beds of the upper Oolite, numerous fossil plants are found. Peuce Lindleyana is one of the Coniferæ of the lower Oolite. Beania ([Plate II. Fig. 2]) is a Cycadaceous fossil from the Oolite of Yorkshire (Carruthers, Geol. Mag. vi. 91). Araucarites sphærocarpus (Figs. 83, 84, 85) is found in the inferior Oolite, and separate scales of Araucarian fruits occur in the Oolitic shales of Yorkshire (Araucarites Phillipsii, [Plate II. Fig. 11]), and in the "slate" at Stonesfield (A. Brodiei, [Plate II. Fig. 10]). The upper Oolite at Portland contains an interesting bed, about a foot in thickness, of a dark brown substance. This is the Dirt-bed (Fig. 86) made up of black loam, which, during the Purbeck period, formed a surface soil which was penetrated by the roots of trees, fragments of whose stems are now found in it fossilised. These consist of an assemblage of silicified stumps or stools of large trees, from 1-3 feet high, standing in their original position, with the roots remaining attached to them, and still penetrating the earth in which they grew. Besides the erect trunks many stems have been broken and thrown down, and are buried in a horizontal position in the bed. They belong to Coniferæ and Cycadaceæ. One of these is Mantellia nidiformis, shown in Fig. 87. Carpolithes conicus and C. Bucklandi are fruits found in the Oolite. Some look upon them as fruits of palms.
Fig. 88. Fig. 89.
Fig. 88. Kaidacarpum ooliticum, Carr., fruit of a fossil allied to Pandanaceæ, from the great Oolite near Northampton.
Fig. 89. Pandanus odoratissimus, Screw-pine, with adventitious roots.
Several species of Pandanaceous fruits have been found in Oolitic strata. Buckland described one of them as Podocarya, which is remarkable, as it consists of a single but many-seeded drupe. To another form, more nearly allied to the existing plants, Carruthers has given the name Kaidacarpum, and has described three species. These fruits are made up of a large number of single-seeded drupes. The species figured (Fig. 88) is from the great Oolite, near Northampton. In Fig. 89 a representation is given of one of the Pandanaceæ, the screw-pines of the present day.