ROCK-FORMATIONS. 23 



molten lava. The earthquake also, which is but another 

 manifestation of volcanic agency, is ever breaking up the 

 rocky crust here raising the sea-bed into dry land, and 

 there submerging the dry land beneath the ocean here 

 rending and fissuring, and there producing inequalities and 

 varieties of surface. Whatever is worn and wasted from 

 one portion of the crust is laid down in another ; there is 

 nothing lost ; but the interchanges and variations are inter- 

 minable. The crust we dwell upon stable and enduring 

 as we are accustomed to regard it is thus a thing of inces- 

 sant change, protean in its superficial aspects, and ever- 

 shifting in its terraqueous arrangements. 



If the earth's crust be thus continually worn away in 

 one district and reconstructed in another, some portions 

 like the lavas of Etna and the delta of the Gauges must be 

 comparatively recent, and others like the Grampian Moun- 

 tains and the coal-fields of Britain of vast antiquity. In 

 the former instances, the lavas and mud-islands are forming 

 beneath our observation ; in the latter, the formative pro- 

 cesses have ceased, and no perceptible change has occurred 

 for ages. To arrange the rock-formations of the earth into 

 chronological order is one of the first duties of geology, for 

 without this sequence there could be no history, and a con- 

 nected history of the changes this crust has undergone is 

 the great object of all geological investigation. By a rock- 

 formation is meant the strata that have been deposited in 

 any lake, estuary, or sea-area. The layers of mud, clay, 

 marl, sand, and gravel which have filled up any ancient 

 lake constitute a lacustrine formation ; the sediments that 

 are similarly deposited in estuaries an estuarine formation ; 

 and those deposited in seas, and subsequently upraised into 

 dry land, a marine formation. In course of time, by pres- 

 sure, chemical and other means, sands become sandstones, 

 gravels conglomerates, clayey muds shales, calcareous muds 



