746 ■ REPORT— 1900. 



Report on the Exploration of Irish Caves. 

 See Reports, p. 340. 



MONDAY, SEPTEMBER 10. 



A joint Discussion with Section K on the Conditions under which the Plants 

 of the Coal Period grew was opened by the reading of the following Papers : — 



Flora of the Coal-measures. By R. Kidston. 



Leaving out of consideration for the meantime a few genera of which we 

 possess little or no definite kaowledge, the flora of the Coal-measures consists of 

 Fer72S, Calamites, Li/copods, Splienophyllem, Cordaites, and Conifcrce. 



In genera and species the ferns are probably more numerous than the whole of 

 the other groups, and contain representatives of the Eusporangiate and Lepto- 

 sporangiate members of the class. The Eusporangiate, or those ferns whose 

 sporangia are unprovided with an annulus, were more numerous in the Carboni- 

 ferous period than at present, though in the Coal-measures they do not appear to 

 have been more numerous than the genera with annulate sporangia. Tree ferns, 

 though not very common, are more frequent in the Upper than in the Lower 

 Coal-measures, in the lowest beds of which they seem to be very rare. 



The Calamites are largely represeuted throughout the whole of the Coal- 

 measures, A&terophyllites (^Calamocladus) and Annularia probably being their 

 foliage. 



Lycopods are also very numerous, and are represented by many important 

 genera — Lycopodites, Lepidodendro7i, LejndopJdoios, Bothrodendron, and Siyillaria, 

 with their rhizomes Stiymaria and IStiymariopsis. These genera contributed 

 largely to the formation of Coal. 



The genus Splienojjkyllujn was also frequent during Co.al-measure times, and 

 forms a type of vegetation essentially distinct from any existing group. 



The Gymnosperms are represented by Cordaites, Coniferce, and Cycads. 



The Cordaites had tree-like trunks and long yucca-like leaves. They are 

 plentiful in the Coal-measures, and, like the arborescent lycopods, must have been a 

 prominent feature in a Carboniferous forest-scene. 



The Conifercc, so far as I have seen, are only represented by a single specimen 

 of Walchia from the Upper Coal-measures ; and though Cycads have been discovered 

 in the Upper Coal-measures on the Continent, I am not aware of any British 

 species which can be referred with certainty to this group. 



The Origin of Coal. By A. Strahan, M.A. 



The deposition of the Coal-measures was due to the subsidence of large por- 

 tions of the earth's crust to a depth often amounting to several thousand feet. 

 The subsidence, being unequal, led to the formation of coal-basins, parts of the 

 margins of which are still recognisable. That the intervening areas rose no 

 less rapidly than the basins sank is proved by the vast denudation suflered by the 

 earlier Palaeozoic rocks during the Carboniferous period. 



The subsidence was counterbalanced, during Coal-measure times by sedimenta- 

 tion, for the occurrence of marine beds among deposits of a generally estuarine 

 aspect proves that the surface was maintained at or near sea-level. The 

 Carboniferous sediments consist, in the majority of coal-fields, of marine limestones 

 in the lower part, of marine grits and conglomerates in the middle part, and of 

 estuaro-marine sandstones and shales in the upper part. The sequence is due, 

 firstly, to the admission of the sea to the subsiding areas ; and lastly, to the 

 restoration of level brought ahout by sedimentation and denudation. But there 

 is evidence also of the sedimentation having been more or less spasmodic. Thus 

 the Limestone Series generally consists of repetitions of small groups of strata, 



