ON COAL. 141 



that tlie exchange between pole and equator currents of the aqueous 

 and aerial ocean must have been not only very sluggish hut perfectly 

 regular northeast and southwest currents in the northern hemi- 

 sphere, and northwest and southeast currents in the southern. In 

 proportion as the earth cooled the diversity of temperature between 

 pole and equator became greater and the exchange more rapid. In 

 the meantime the gradual increase in the extent and elevation of con- 

 tinents would introduce still greater diversity. The regular oceanic 

 currents, by impinging upon the continents, are reflected in various 

 directions, increasing still further the diversity of climate. Currents 

 of the air^ too, are no longer only trade winds, but also monsoons, land 

 and sea breezes, &c. These various currents, mingling and contend- 

 ing, produce the infinitely varying winds of the present epoch. But 

 the most important current we have not yet spoken of. Land and sea 

 may be considered the two poles of a circulating apparatus ; water 

 rises in the form of vapor at one pole, passes over through the atmo- 

 sphere, and is condensed on the other in the form of rain, and so back 

 by the rivers to the ocean. The more rapid the condensation the more 

 rapid the evaporation and the more rapid the circulation. Within 

 certain limits, (i. e., until the land is sufficient to condense all the water 

 evaporated from the ocean,) the amount of evaporation and condensa- 

 tion is in proportion to the extent and elevation of the continents. 

 It is evident, then, that in the earlier periods of the earth's history, 

 when the ocean was almost universal, although the air was saturated 

 with moisture, there was comparatively litfle rain ; and that just in 

 proportion as the continents increased in extent and elevation, evapor- 

 ation, and condensation would increase in the same proportion. It is 

 impossible to resist the conclusion, then, that from the earliest jjeriods 

 until now there has been a constant increase in activity and variety of 

 currents of ocean and atmosphere ; of wind and rain ; of cloud and 

 sunshine ; of fountains and rivers ; in fact of all that constitutes the 

 life, variety, and beauty of our beloved earth. 



Thus it appears that at first igneous predominated over aqueous agen- 

 cies. It was this very predominance which caused uncompensated, pro- 

 gressive change — development of the earth as a whole ; for perfect 

 balance is incompatible with developement. But gradually aqueous 

 agencies increased in energy ; the antagonistic forces approached a 

 balance as the earth approached maturity, until at present the balance 

 may possibly be complete. 



In all I have said I have had in view, of course, only the ordinary 

 regular operation of aqueous agencies, or what Mr. Lyell calls " causes 

 now in operation." I say of course, because the extraordinary, irregu- 

 lar operation of these agencies, such as are called ^^ debacles," &c., are 

 too uncertain and hypothetical, not to say improbable, to form the 

 basis of any reasoning whatever. I repeat, then, that during the coal 

 period the ordinary operation of aqueous or degrading agencies must 

 have been more slow than at present. The accumulation of a certain 

 amount of material in a river delta, other things being equal, would 

 require a longer time than now. 



