THE CARBONIFEROUS FLORA. 141 



texture, present in slices of coal, may incline an observer, 

 not having large experience in the examination of coals, 

 to overrate their importance ; and this I think has been 

 done by most microscopists, especially those who have 

 confined their attention to slices prepared by the lapidary. 

 One must also bear in mind the danger arising from mis- 

 taking concretionary accumulations of bituminous matter 

 for sporangia. In sections of the bituminous shales ac- 

 companying the Devonian coal above mentioned, there 

 are many rounded yellow spots, which on examination 

 prove to be the spaces in the epidermis of Psilophyton 

 through which the vessels passing to the leaves were 

 emitted. To these considerations I would add the fol- 

 lowing, condensed from the paper above referred to 

 (p. 139), in which the whole question of the origin of 

 coal is fully discussed : * 



1. The mineral charcoal or ' mother coal ' is obviously 

 woody tissue and fibres of bark, the structure of the va- 

 rieties of which, and the plants to which it probably be- 

 longs, I have discussed in the paper above mentioned. 



2. The coarser layers of coal show under the micro- 

 scope a confused mass of fragments of vegetable matter 

 belonging to various descriptions of plants, and includ- 

 ing, but not usually in large quantities, sporangites. 



3. The more brilliant layers of the coal are seen, 

 when separated by thin laminse of clay, to have on their 

 surfaces the markings of Sigillarim and other trees, of 

 which they evidently represent flattened specimens, or 

 rather the bark of such specimens. Under the micro- 

 scope, when their structures are preserved, these layers 

 show cortical tissues more abundantly than any others. 



4. Some thin layers of eoal consist mainly of flat- 

 tened layers of leaves of Gordaites or Pychnophyllum. 



5. The Stigmaria underelays and the stumps of 



* See also "Acadian Geology," 2(1 ed., pp. 138, 461, 493. 

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