24 THE CRUST WE DWELL UPON. 



limestones, muds largely impregnated with iron ironstones, 

 and vegetable masses coals ; and it is in this way that the 

 sediments of former lakes and estuaries and seas have 

 become the rocky strata that now constitute the crust of 

 the globe. As might be expected, there will often be every 

 degree of admixture among these rocky strata, just as there 

 is every degree of admixture and impurity among the sedi- 

 ments of existing seas and estuaries. There will be sand- 

 stones argillaceous, and sandstones calcareous ; shales bitu- 

 minous, shales calcareous, and shales ferruginous; limestones 

 argillaceous, and limestones siliceous ; coals so pure as to 

 burn away without leaving scarcely a trace of ashes, and 

 others so stony as to be altogether unfit for fuel. The solid 

 crust is, indeed, mainly made up of mixed rocks that is, of 

 arenaceous (sandy), argillaceous (clayey), calcareous (limy), 

 siliceous (flinty), bituminous (coaly), ferruginous (iron- 

 impregnated), and other similar compounds ; but whether 

 these rocks be sedimentary sandstones, grits, conglomerates, 

 shales, limestones, and ironstones, or fire -formed lavas, 

 greenstones, basalts, and granites, the great object of 

 geology is to distinguish between the older and newer, 

 to arrange them in chronological order, and so arrive, if 

 possible, at a knowledge of the geographical conditions 

 which accompanied their formation. Nor is this endea- 

 vour in the least chimerical or uncertain ; for as was well 

 remarked by Humboldt, now nearly half a century ago 

 " The superposition and relative age of rocks are facts sus- 

 ceptible of being established immediately, like the structure 

 of the organs of a vegetable, like the proportions of ele- 

 ments in chemical analysis, or like the elevation of a moun- 

 tain above the level of the sea. True Geognosy makes 

 known the outer crust of the globe, such as it exists at the 

 present day. It is a science as capable of certainty as any 

 of the physical descriptive sciences can be." 



