TIIK TLOIIA OF THE COAL FOKMATION. 471 



The number of coals reckoned in this coal-field may vary according 

 to the manner in which the several layers are grouped ; but as arranged 

 in the sectional list given in a previous chapter it amounts to eighty- 

 one in all. Of these, 23 are found in Division 3 of Logan's section, 

 being the upper member of the Middle Coal formation ; 49 are found 

 in Division 4 of Logan's section, being the lower member of the Middle 

 Coal formation ; 9 occur in Division 6 of Logan's section, or in the 

 equivalent of the Millstone-grit. In the latter group few of the coals 

 were sufficiently well exposed to enable a satisfactory examination to 

 be made. I have grouped the remains under three heads — External 

 Forms of Plants, Microscopic Structure of Plants, and Animal Re- 

 mains — and have arranged the forms under each in the order of their 

 relative frequency of occurrence. No mention is made of Stigmaria, 

 which occurs in nearly every coal or its underclay. 



The following are the conclusions, based on the above table and on 

 examinations of the Coal of Pictou and Sydney : — 



" 1. With respect to the plants which have contributed the vegetable 

 matter of the coal, these are principally the SigillaricB, with Cor- 

 daites, Ferns and Calamites. With these, however, are intermixed 

 remains of most of the otlier plants of the period, contributing, though 

 in an inferior degree, to the accumulation of the mass. This conclusion 

 is confirmed by facts derived from the associated beds, — as, for instance, 

 the prevalence of Stigmaria in the underclays, and of SigillaricB and 

 Calamites in the roof-shales and erect forests. 



" 2. The woody matter of the axes of S/'gillarice and Calamitea: and 

 of coniferous trunks, as well as the scalariform tissues of the axes of 

 the Lepidodendrece and Ulodendrea', and the Avoody and vascular 

 bundles of ferns, appear principally in the state of mineral charcoal. 

 The outer cortical envelope of these plants, together with such portions 

 of their wood and of herbaceous plants and foliage as were submerged 

 without subaerial decay, occur as compact coal of various degrees of 

 purity ; the cortical matter, owing to its greater resistance to aqueous 

 infiltration, affording the purest coal. The relative amounts of all 

 these substances found in the states of mineral charcoal and compact 

 coal depend principally upon the greater or less prevalence of subaerial 

 decay, occasioned by greater or less dryness of the swampy flats on 

 which the coal accumulated. 



" 3. The structure of the coal accords with the view tliat its ma- 

 terials were accumulated by growth, without any driftage of materials. 

 The SigillaricB and Calamitea'^ tall and branchless, and clothed only 

 with rigid linear leaves, formed dense groves and jungles, in which 

 the stumps and fallen trunks of dead trees became resolved by decay 



