856 REPORT -1901. 



4. Roots. 



Nothing- new lias been observed in the roots of the ^Marattiacere. Tn ihe 

 roots of Dana-a simjiUcifulia there is what might be called a fibrous pith, which 

 is early differentiated, even before the main mass of the xylem has begun to be 

 lignified. 



4. A Chaiyter of Plant-fivohition : Jurassic Floras. 

 By A. C. Seward, F.R.S. 



From the cliffs on the Dorsetshire coast to the moorlands and headlands of 

 East Yorkshire England is traversed diagonally by a band of Jurassic strata, and 

 outlying patches of Jurassic rocks occur ia West Somerset, Gloucestershire, 

 Worcestershire, Cumberland, and elsewhere. Sediments of the same age occur 

 also in Siitherlandshire, in the islands of Skye and Mull, and in other parts of Scot- 

 land. After the filling up of the inland lakes of the Triassic period, the land 

 gradually subsided and was invaded by a shallow sea in which a thin band of 

 Rhffitic sediments was deposited in the Uritish area. The vegetation of this 

 period is represented by the rich floras of Scania, Franconia, and other districts, 

 but in Britain by a few meagre and imperfect records. The rocks formed on the 

 floor of the deeper Liassic sea have afforded several Cycadean fronds and fragments 

 of coniferous trees drifted from neighbouring land. From the estuarine beds 

 intercalated in the series of marine strata of the Oolitic period, an abundant flora 

 has been obtained from Yorkshire and elsewhere. The roofing slates of Stones- 

 field, described by Plot in his ' Natural History of Oxfordshire ' in 1677, have 

 yielded numerous fragments of plants, which may be the relics of the vegetation 

 of an island in the Jurassic sea. From the Oxford clay, Corallian beds, and the 

 Kimeridge clay a comparatively small number of plants have been obtained, while 

 from the overlying Portlandian and Purbeck series the well-known Cycadean 

 stems and the abundance of silicified coniferous wood demonstrate the prominent 

 role played by gytunospermous plants in the vegetation of the land, which had 

 gradually encroached on the Jurassic ocean. Finally, a rich flora, preserved in 

 the freshwater Wealden sediments, affords a striking proof of the slow change 

 in the character of the vegetation since the Inferior Oolite period. 



The chief features in the floras ranging from the Rhretic to the Wealden are 

 briefly described ; an attempt is made to determine the dominant types during this 

 long succession of stages in the earth's history, and to estimate the progress of 

 plant-evolution from the close of the Triassic period to the appearance of 

 Angiosperms in rocks of Lower Cretaceous age. 



5. On the Structure and Origin of Jet. By A. C. Seward, F.R.S, 



The hard jet of Whitby appears to have been used in Britain in pre-Roman 

 days; it is alluded to by Oaedmon and mentioned in 1350 in the Records of 

 St. Hilda's Abbey. It was formerly extensively mined in the cliffs of the York- 

 .shire coast, near AVhitby and elsewhere ; in Eskdale, Danby Dale, and in several 

 of the dales that intersect the East Yorkshire moorlands. The hard jet occurs in 

 the Ammonites serpentinus zone of the Upper Lias, frequently in the form of 

 flattened masses or layers, which in rare cases have been found to reach a length 

 of 6 feet. Parkinson in his ' Organic Remains of a Former World ' (1811) speaks 

 of jet, in some cases, as pure bituminised vegetable matter, and the majority of 

 writers regard it as having been found as a product of alteration of plant tissues. 

 On the other hand it has been described as ' the result of the segregation of the 

 bitumen' in the intervals of the jet shales, which has sometimes formed pseudo- 

 morphs after blocks of wood.\^ The author has recently examined several sections 

 of Yorkshire jet in the British Museum, which he believes demonstrate the origin 

 of this substance from the alteration of coniferous wood and, in part -it least, of 

 wood of the Araucarian type. 



' Tate'and Blake, The Yorlisldre Lias, 187C. 



