THEIR CHRONOLOGICAL STAGES. 267 



of the flinty shields of polycistinae and diatomacese. The 

 foraminifers and polycistines belong to the animal kingdom, 

 the diatoms to the vegetable ; but all require the higher 

 powers of the microscope for their study, and when ob- 

 servers speak of miles of diatomaceous earths, and thou- 

 sands of miles of foraminiferal muds, the mind is utterly 

 impotent to grapple with the conception of the numbers 

 of individual organisms that must have contributed their 

 quota to the aggregate amount. But so it often happens 

 in nature, that the most gigantic results are brought about 

 by the minutest agents, and by the most imperceptible 

 stages the main conditions being incessant activity and 

 unlimited duration. 



The last of the Eecent Formations that fall within the 

 scope of our Sketch are the Volcanic, or those that have 

 arisen from the substances discharged by modern volcanoes. 

 These formations will consist mainly of lava in its different 

 varieties, of tufa or compacted dust and ashes, of sconce 

 or cindery and slaggy matter, of pumice, obsidian, lapilli, 

 and agglomerates that have accumulated by the washing 

 together of heterogeneous volcanic products. The pro- 

 ducts of every active volcano are more or less an epi- 

 tome of those of all the rest ; for though some, like those 

 of the Andean cones, consist mainly of light cindery dis- 

 charges, the great majority are admixtures, in varying 

 sheets and masses, of all the substances above enume- 

 rated. Kow a shower of dust and ashes blown aloft and 

 scattered over one or other side of the mountain ; now an 

 overflow of lava slowly wending its way for miles down 

 the rugged slopes; now explosive discharges of slag and 

 cinders ; and anon some gigantic lava-stream enveloping 

 the whole in its rocky mass again to be overlaid by re- 

 peated successions of similar materials. Etna, Vesuvius, 



