142 LECTURES 



CLIMATE OF THE COAL PERIOD. 



It is probable, from what evidence we have on this subject, that 

 the climate of the coal period was characterized by greater warmth, 

 greater humidity, and greater uniformity than now obtains, and that 

 the air was more highly charged with carbonic acid. Of the greater 

 warmth of the climate we have evidence in the astonishing luxuriance 

 and universal tropical character of the vegetation of the period. One 

 of the most marked peculiarities of the flora of coal everywhere is the 

 great relative abundance of ferns and fern allies. In the present flora 

 of Great Britain the ratio of ferns to flowering plants is about 1 to 35, 

 while in the coal flora of the same country nearly one-half of all the 

 known plants are ferns. In the American coal flora the ])roportion 

 of ferns is said to be still greater. That this abundance of ferns indi- 

 cales a tropical climate is shown by the fact that in the existing flora, 

 out of about 1,500 known species of ferns, 1,200 are confined to the 

 tropics^ and as we pass from the equator towards the poles the propor- 

 tion of ferns, steadily diminishes. The same may be said with refer- 

 ence to the club-mosses. It is worthy of remark, too, that although 

 conifers are abundant now all over the earth's surface, still those most 

 nearly allied to the conifers of the coal — such, for instance, as the 

 araucaria and salisburia of the present day — are found only in tropical 

 regions. Now, during the coal period, this tropical vegetation extended 

 as far as 75° north latitude. Tree ferns and gigantic club-mosses 

 covered the spot now occupied by the Mellville island. The evidence 

 of remarkable humidity is no less satisfactory, for it is only in warm, 

 7noist climates that ferns and club-mosses grow in the greatest abun- 

 dance and luxuriance. On some islands in the tropics and in the 

 south seas the abundance of ferns even approaches that of the coal 

 flora. In fact, as a condition of the growth of these plants, moisture 

 seems even more necessary than heat. 



It has been objected to the greater heat of the climate, that coal was 

 evidently formed by accumulation of carbonaceous matter in situ as 

 now in peat bogs, and that peat bogs are found only in cool climates. 

 The answer to this objection is not diflicult. It is not the heat 

 immediately, but the resulting capacity for moisture, or, in common 

 language, dryness of the air of the tropics, which under ordinary cir- 

 cumstances prevents the preservation of carbon. The air is not so 

 constantly at or near the point of saturation. Fogs, and mists, and 

 clouds are not so constant as in cooler climates. But we have sup- 

 posed greater humidity as well as heat during the coal period. Under 

 these circumstances, there is no reason why peat should not accumu- 

 late. We see proof of this in the peat swamps at the mouth of the 

 Mississippi. Here we find peat accumulating in great abundance in 

 a climate which is yet very warm ; and we have already seen that it 

 is in such peat swamps, rather than in the bogs of cooler climates, 

 that we are to look for analogies with the peaty accumulations of the 

 coal period. The enormous extent of these peat swamps becomes in its 

 turn an additional ])roof of the great humidity of the climate. 



The uniformity of climate — i. e. the comparatively equable distri- 



