242 GEOLOGICAL DESCRIPTION OF THE EARTH*S CRUST. 



surface of our planet would have consisted of horizontal 

 strata, regularly superimposed one over the other. Deprived 

 of our mountain chains, the declivities of which may be 

 said to reflect, in the picturesque gradation of the different 

 vegetable forms with which they are clothed, the scale of 

 diminishing atmospheric temperature from their base to their 

 summits, the only features of variety in the disposition of 

 the ground would have been the occasional presence of 

 ravines hollowed out by the feeble erosive force of currents 

 of fresh water, and slight eminences of transported detritus 

 due to the same cause. Prom pole to pole, under every 

 region, continents would have presented to the eye the 

 dreary uniformity of the llanos of South America, or the 

 steppes of Northern Asia ; the vault of heaven would have 

 everywhere appeared to rest on the unbroken plain, and the 

 stars to rise and set as on the horizon of the ocean. Such 

 a state of things, however, cannot have had a long duration 

 even in the primitive world, for at all periods subterranean 

 forces have exerted their modifying influence. 



Sedimentary strata have been either precipitated or depo- 

 sited from liquids, according as the materials which con- 

 stitute them were chemically dissolved or mechanically 

 suspended. But when earths that have been dissolved in 

 fluids containing carbonic acid are separated, their descent 

 during precipitation and accumulation in strata require to be 

 regarded as a true mechanical process; a consideration of some 

 importance in respect to the envelopment of organic bodies in 

 fossiliferpus limestone. The oldest sedimentary strata of the 

 transition and secondary series were probably formed from 

 water of high temperature, at a time when the heat of the 

 upper surface of the globe was still very considerable. In 



