382 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



the productive coal measures that resulted from their activities, is 

 the widespread uniformity of physical conditions prevailing at that 

 time, which facilitated the formation of coal. 



Coal is found somewhere in the world in all geological periods 

 subsequent to the Devonian, but scarcely anywhere or at any time 

 was nature's balance so delicately adjusted that such a percentage 

 of the. contemporaneous vegetation escaped oxidation, accumulated 

 over long periods, and was preserved. The impression that these 

 coal swamps were like modern peat bogs, or that they covered thou- 

 sands of square miles along the coasts of Appalachia is not borne 

 out by the extent and lateral variation of the coal seams and asso- 

 ciated sandstones and shales, and it is becoming increasingly clear 

 that while favorable conditions prevailed over wide areas, individual 

 seams are usually restricted in extent ; some representing low coastal 

 swamps, others valley swamps, and others lagoonal and lake deposits 

 of drifted materials and a rain of spores and forest litter. 



That the humidity w T as high and the sky cloudy is reasonable, but 

 the evidence does not support the various theories that the sun was 

 normally hidden or that the atmosphere was dense with carbonic 

 acid gas. Nor was the heat torrid. That there was not the latitudi- 

 nal or seasonal variation that exists to-day is shown by the cosmo- 

 politanism of the floras and the normal absence of growth rings in 

 the coniferous woods. It has been suggested that our knowledge of 

 Carboniferous floras is confined to those which grew on lowlands 

 adjacent to the regions of coal accumulation, and while this is true, 

 it should be borne in mind that lowland and upland floras of any 

 period differ merely in generic and specific types, and there is no 

 reason to suppose that unknown orders or nascent phyla inhabited 

 the Carboniferous uplands. 



We may draw a generalized picture of a Carboniferous or Per- 

 mian swamp as densely forested with clumps of slender, graceful 

 Calamites — scouring rushes enlarged to fifty times the size of the 

 modern ones — growing in and about the margins of the water; mas- 

 sive columnar Sigillarias with their persistent crown of needle- 

 like leaves and their shallow, spreading roots; tall Lepidodendrons 

 with their branched and evergreen crowns; lofty cordaites with their 

 large pendent, evergreen leaves; large clumps of Marattiaeeous and 

 other ferns, slender-stemmed Sphenophyllums clambering over and 

 among the other vegetation, and a variety of seed ferns everywhere — 

 some with long, slender stems like Lyginopte»is dependent on the 

 jungle for support, while others with massive stems like Medullosa 

 quite capable of taking care of themselves in the universal upward, 

 light-seeking growth. (See fig. 14.) 



Chronologically, many of the Devonian types lingered into the 

 Lower Carboniferous, during which time Archaeopteris and Psilo- 



