84 THE PRIMARY PERIODS. 



the least altered and most resemble those still living in 

 the district. These propositions are so evident that it 

 would be little else than waste of time to attempt a more 

 detailed explanation. 



"We have, then, in the crust of the earth some formations 

 very recent and others very ancient so recent that the 

 formative processes are still in action, and so ancient that 

 it requires all the appliances of modern science to say how 

 they were formed, and to determine the nature of the fossil 

 organisms they contain. The great plain of China and the 

 delta of the Mississippi, for instance, are of recent growth, 

 and indeed still in course of accumulation ; and by a parity 

 of reasoning we may readily perceive that the prairies of 

 North America and the pampas of South America, though 

 somewhat older, are still of comparatively recent formation. 

 Everything connected with these accumulations mineral 

 composition or imbedded organisms has an air of recent- 

 ness about it, and is readily intelligible ; but it is different 

 with the sandstones and limestones that lie beneath, whose 

 structure has been altered, and whose fossils have been 

 mineralised and rendered obscure. These sandstones and 

 shales and limestones were no doubt at one time loose sands 

 and clays and calcareous muds, but pressure, chemical ac- 

 tion, and other metamorphic forces, continued through long 

 ages, have converted them into solid strata, and this conver- 

 sion has been intensified, for the most part, according to 

 their relative dates. As with the stratified or sedimentary 

 formations, so also with the igneous. Such hills as Hecla, 

 Etna, and Teneriffe readily reveal their history; but the 

 Alps, the Pyrenees, and the Grampians, whose internal 

 fires have been long since extinguished, whose rocks have 

 become more crystalline, whose heights have been repeatedly 

 beneath the waters, and from which all the loose volcanic 

 matters have been washed and worn away, present a very 



