VII. The Dictyoxylon cortex of lycopodiales as a constituent of coal

1. The condition of preservation of plant remains in coal, especially in the bright bands, has hitherto made the identification of definite tissues a task of great difficulty. The structure has been so obliterated or obscured that in thin sections the material usually appears almost homogeneous. Such bands have a vitreous lustre and conchoidal fracture, and have been called by Dr. M. C. Stopes (1)vitrain . The process of vitrainisation, whatever its nature, is seldom complete, and structure may exist from mere traces up to a degree of preservation sufficient to give a striated appearance and silky lustre to the fracture, which Dr. Stopes has characterised as that ofclarain . 2. The application to coal of the metallographic method of polishing, etching and examining by reflected light, made by Dr. H. Winter (2) and developed by the writer (3), has revealed the presence of structure where thin sections fail to show it. The technique is simple. The polishing follows the lines originally laid down by Sorby, and the etching fluid is a boiling saturated solution of chromic acid, containing a little sulphuric acid. The method is applicable to all kinds of coal, even anthracite. It has the advantage that sections in three determined planes at right angles may be made at one spot, which is difficult or impossible with thin sections, and yet essential to the interpretation of the structure of a plant fragment.