INTRODUCTION. 9 



quent denudation of the covering mass, and that these beds served later on 

 as moulds for succeeding deposits, exactly as in the case of the tracks of 

 crabs described by Nathorst. A similar explanation may be given of the 

 leaves of Nymphaeaceae from Tertiary formations adduced by Saporta l in 

 his elaborate account of this ' fossilisation en demi-relief ;' in the case of the 

 fragments of the rhizomes of the same plants shreds of the epidermis may have 

 been set free by the rotting of the plant, and been floated into the place 

 where the fragment was being preserved. A careful examination of this 

 question will be found also in the introduction to Delgado's work 2 . 



Coal-seams, or coal-streaks as they are termed when they are only 

 slightly developed, are a very special kind of incrustation. The only differ- 

 ence between them and the impressions of plants hitherto described is, that 

 in their case not single fragments but huge deposits of vegetable remains 

 are incrusted together with conversion of their substance into coal, and are 

 inclosed in a mass of material which hardens into stone. The opposite 

 faces in the impression are in this case the roof and floor of the seam, and 

 parts of plants which happened to lie on the surface of the seam, and which 

 cannot now be distinguished in the coal itself, are not unfrequently shown in 

 well-preserved moulds in the bounding surface, especially in the roof. Many 

 instances of the kind are adduced by Goppert 3 , who found in the roof of 

 coal-seams in Upper Silesia impressions of one side of large tree-stems which 

 could sometimes be traced for a length of several metres. 



It is in a very small portion only of the coal-seams that their origin from 

 fragments of higher plants can be readily seen at the first glance. This is 

 most frequently the case in the brown coals of the more recent formations ; 

 it occurs much more rarely in the coals of the mesozoic and palaeozoic ages. 

 An excellent example is afforded by the coal-beds of Central Russia. 

 We learn from Goppert 4 that at Malowka in the Government of Tula, 

 besides more compact kinds of coal there is one which consists entirely of 

 flakes, like sheets of paper, loosely united together and pierced with little 

 holes, and that closer examination shows that these flakes are shreds of 

 the cuticle of Lepidodendreae (Lepidodendron tenerrimum, Eichw., Bothro- 

 dendron punctatum, Grand' Eury). They are so slightly coherent that 

 they can be blown apart by the wind. Nevertheless these coals belong to 

 the oldest formation, which generally contains beds of the most compact 

 coal ; for geologists, though they differ in opinion on particular points, 

 agree in placing them on the horizon of the Carboniferous limestone. 



The older coals, the pit-coals, are usually compact, more or less 

 schistose and tolerably homogeneous, dull or bright, and of a black colour. 

 From their outward appearance they would be taken for simple minerals, 



1 de Saporta (12). 2 Delgado (1). 3 Goppert ,14). * Ibid. (13). 



