THE PLOKA OP Till: CoAl. FORMATION. 193 



3. The greater part of the species have their head-quarteri in tin- 

 Middle Coal formation, and scarcely any species appear in the Upper 

 Coal formation that arc not also found in the former. The Lou 



Coal formation, on the other hand, seems to have a few peculiar sp& 

 not found at higher levels. 



4. The characteristic species of the Lower Coal formation arc Lepi- 

 dodendron corrugatum and Cyclopteris Acadica, both of which seem 

 to he widely distributed at or near this horizon in Eastern America, 

 while neither has yet been recognised in the true or Middle Coal 

 measures. In the Upper Coal formation Catamites Suckovii, Anmtr 

 laria sphenophylloides, tfphenojjhyllum emarf/inatuni, CordaiUa ihnpl 

 Alcthopteris nervosa, muricata, etc., Pecupteris arborescent, J', ab- 

 breviata, P. rigida, Neuropteris cordata, Dadoxylon mater/arum, 

 Lcpidophloios parvus, Sigillaria scutellata, are characteristic plants, 

 though not confined to this group. 



5. In the Middle Coal formation and in the central part of it, near 

 the greater coal seams, occur the large majority of the species of 

 Sigillaria, Calamites, Lcpidodendron, and Ferns ; some of the species 

 ranging from the Millstone-grit into the Upper Coal formation, while 

 others seem to be more narrowly limited. It is to be observed, how- 

 ever, that, as we leave the central part of the system, the total number 

 of species diminishes both above and below, and that it is only in those 

 beds which hold large numbers of plants in situ or nearly so, that we 

 can expect to find a great variety of species, and especially the more 

 delicate and perishable organisms. 



It is also quite observable in the Joggins section, that while some 

 beds, in the same part of the system, supported tiigillarice, others 

 carried Calamites, others mixtures of these with other plants ; so that 

 differences of soil, moisture, etc., frequently cause neighbouring beds 

 to be more dissimilar in their fossil contents than others much more 

 \\ idely separated. These local and temporary differences must always 

 have occurred in the deposition of the coal measures, and should not 

 be confounded with those general changes which are connected with 

 lapse of time. 



Additional Note on Vegetable Structures in Goal. 



In the foregoing pages reference has frequently been made to the 

 existence of distinct vegetable structures in coal; and any ordinary 

 observer may satisfy himself of this by closely inspecting the surfaces 

 of a lump of the mineral with the aid of a bright li.^ht and a magni- 

 fying glass. But the microscope reveals a world >•[' wonderful tissues 

 in coal, as perfect as if they had only yesterday formed parts of living 

 plants; and as I have devoted many hours of patient labour to the 



