12 STUDIES IN FOSSIL BOTANY 



section. Other substances, such as calcium sulphate 

 and phosphate, have played a part in petrifaction, and 

 so too, among organic products, has amber. In a few 

 cases, fine clay has penetrated vegetable substances 

 sufficiently to cause an effectual petrifaction ; one 

 important specimen, to which we shall have occasion 

 to refer, is preserved in this way (see Chapter IV. 

 p. 107). 



The calcified remains of the English Coal-measures 

 occur largely in the form of calcareous nodules or coal- 

 balls, stony masses which are found in certain localities, 

 especially in Yorkshire and Lancashire, in the actual 

 productive coal-seams, and also in the "roof" above 

 them. The former represent, as it were, parts of the raw 

 material of coal which have been saved by petrifaction 

 from carbonisation, and have consequently retained their 

 structure. These nodules are literally crowded with 

 vegetable remains of all kinds. The calcareous material, 

 of which these nodules are formed, may have been derived 

 from the shells of marine Mollusca, which lived and 

 died on the old forest ground when sunk once more 

 beneath the sea, or the lime normally contained in sea- 

 water may have sufficed. 



We can form a good idea of the sort of material 

 which these nodules contain, if we notice the deposits 

 of vegetable debris left on the banks of a tidal river. 

 There we find miscellaneous fragments of plants heaped 

 together in utter confusion, bits of reeds and rushes, 

 rhizomes of water-lilies and aquatic grasses, twigs and 

 scraps of bark from river-side trees, seeds, nuts, and 

 cones. If we imagine a handful of such a conglomera- 

 tion, saturated and fixed by some petrifying substance, 



