C.—GEOLOGY 63 
During the years between 1831 and 1858 many sections of fossil plants 
_ were made, and collections formed. One of these, including Nicol’s collec- 
tion, was in the possession of Mr. Bryson, Edinburgh. Sections of 
minerals, as thin as ;45th inch, had also been prepared, but the possibilities 
of the method were not realised until Sorby published his paper On the 
microscopic structures of crystals.** In this work he says that, while on a 
visit to Edinburgh, he saw the ‘ excellent collection of “ fluid-cavities ” 
in the possession of Mr. Alexander Bryson of Edinburgh, who told me he 
had found some in the granite of Aberdeen. I immediately perceived that 
the subject could not fail to lead to valuable results when applied to 
geological enquiries.’ To pursue the subject, thin sections of rock were 
required, and from this beginning sprang the study of microscopic 
- petrology. 
A second development from the discovery of the internal structure of 
- fossil plants is the biological aspect. Witham’s pioneer work was followed 
_ by numerous monographs, and larger works like Lindley and Hutton’s 
_ Fossil Flora of Great Britain, These generally dealt with both incrusta- 
tions and petrifactions. But there were wide differences of opinion in 
regard to the correlation of parts and of relationships. Writing in 1871, 
_ Williamson says of Calamites that Lindley and Hutton had given correct 
illustrations of the relation of root and stem, ‘ and yet for years afterwards 
some of their figures re-appeared in geological text-books in an inverted 
position, the roots doing duty as leaves ; so far was even this elementary 
point from being settled.’ + Brongniart also believed that there were two 
distinct types of Calamites, the one cryptogamic, the other gymnospermous. 
Similar uncertainties existed in regard to other forms, and it was only 
after much patient research that the present position was established. 
One of the most important of results has been the differentiation of the 
great group of the Pteridosperms, and the recognition of their abundance 
‘in Upper Paleozoic times. The story is one of interaction of results 
obtained partly from incrustations and partly from petrifactions, while 
the later results, at any rate, have only been obtained by improvements in 
technique dealing with incrustations. In 1866 Binney described Lyginop- 
teris (Dadoxylon) oldhamium as a plant with gymnospermous affinities. 
_* It evidently belonged to the genus Pinites of Witham, since changed by 
Bendlicher and Brongniart into Dadoxylon.’*> Williamson redescribed 
the plant in 1873, and drew attention to its fern-like and lycopodiaceous 
characters, but not the gymnospermous features. Later, in 1887,26 he 
concluded that Lyginopteris (Lyginodendron) ‘ belongs to the group of 
ferns’ ; but, in the same paper, speaking of Lyginopteris and Heterangium, 
_he stated ‘ possibly they are the generalised ancestors of both Ferns and 
‘Cycads....’ Stur, in 1883, on negative evidence—the non-occurrence 
of sporangia on the fronds—excluded certain of the fern-like incrustations 
in Carboniferous rocks from the fern group, and referred them to cycads. 
> % Q.7.G.S., vol. xiv, p. 454 (1858). 
_ #4 Williamson on the Organisation of the Fossil Plants of the Coal Measures, 
Pt. I, No. 1, Phil. Trans. Roy. Soc., 1871. 
_ *% Binney, Proc. Lit. and Phil. Soc., Manchester, vol. 56. Read, 1866. 
_ *6 Williamson, Phil. Trans. Roy. Soc., p. 299 (1887). 
