IGNEOUS AGENCIES. 45 



the siliceous mud of most of our existing estuaries.* The 

 lime and the flint dissolved by springs from the crust of the 

 earth, and borne down by streams and rivers to the ocean, is 

 thus secreted by vital agency and once more converted into 

 solid rock-matter. Nothing is lost; it may change its 

 shape and pass out of sight for a while, but in the long-run 

 it will reappear, altered it may be in form, but essentially 

 the same in substance. Every particle of matter obeys a 

 ceaseless round of change. Now in the crystalline and 

 independent gem, at another time as a constituent of the 

 solid rock ; now dissolved in the limpid waters, at another 

 time built up in the structures of plants and animals ; now 

 scattered abroad as the decaying exuviae of life, and once 

 more collected into compact and rocky strata. 



The Igneous agents generally exert themselves with signal 

 force and marked effects, and yet in some instances their most 

 gigantic results are brought about by stages that are almost 

 imperceptible. The most notable instance of their opera- 

 tion is perhaps in the volcano, which in course of ages piles 

 up its alternate discharges of dust and ashes and lava till it 

 assumes the lofty proportions of an isolated mountain like 

 Etna, or stretches away in long ranges like the fiery cones 

 of the Andes. Whether exerting itself on land or rising 

 up from the depths of the ocean, the volcano is one of the 

 most important modifiers of the earth's crust ; and it acts 

 partly by upheaval, partly by accumulation, and partly by 

 fusing and reconstructing rock-matter in the interior, and 

 by bringing it once more to the surface. In this function it 



* Of the microscopic organisms the foraminifers, polycystines, dia- 

 toms, and desmids that stand, as it were, on the confines of Life, the 

 two former belong to the animal kingdom and the two latter to the vege- 

 table. The foraminifera secrete calcareous matter, the polycystines and 

 diatoms siliceous, and the desmids no appreciable quantity of either. 

 The foraminifera and diatoms, along with the coral-polypes, may there- 

 fore be regarded as the main microscopic rock-builders. 



