FUNDAMENTAL CLASSES OF EOCKS. 237 



only a faintly reflected image of that which took place in 

 the early chaotic period of more energetic activity, under 

 very different conditions of pressure, and a far higher tem- 

 perature of the more extended and vapour-loaded atmo- 

 sphere, as well as of the crust of the earth. The vast fissures 

 which were then open in the solid portions of the crust have 

 been since closed by the elevation of mountain chains pro- 

 truded through them, or filled up by veins of granite, porphyry, 

 basalt, and melaphyre. At the present period of the globe 

 there remain only, on an extent of the size of Europe, four 

 volcanoes, or openings through which ignited masses may 

 issue ; whereas formerly, channels of communication between 

 the molten interior and the atmosphere existed at almost 

 every part of the thinner and much fissured crust of the globe. 

 Gaseous exhalations, rising from very unequal depths, and 

 bringing with them different chemical substances, gave 

 great activity to the processes of plutonic formation and of 

 metamorphic action. In like manner, in the case of sedi- 

 mentary formations, the beds of travertin which are now in 

 daily course of deposition from cold and warm springs and 

 river water, near Rome, and near Hobarton in Yan Diemen 

 Island, afford but a feeble representation of the formation 

 of the earlier mineral strata. On the coasts of Sicily and of 

 the Island of Ascension, and in King George's Sound in 

 Australia, small banks of limestone, of \vhich some parts are 

 scarcely inferior in hardness to Carrara marble, ( 243 ) are in 

 course of gradual formation by our present seas, under the 

 influence of processes which have not' yet been sufficiently 

 investigated, by means of precipitation, accumulation by 

 drift, and cementation. On the coasts of the West India 

 Islands, these formations of the present ocean contain pot- 



