253 Q30L03ICAL DESCRIPTION 1 OP THE EA.RTH*S CRUST. 



sharply angu.hr masses of fragments. Conglomerates and 

 breccias, in their widest acceptation, present the characters 

 of a double origin. Their mechanical constituents have 

 not been accumulated solely by the action of the sea, 

 or by streams of fresh water, and there are some of these 

 rocks to the formation of which the action of water has 

 not contributed. "When basaltic islands or trachytic 

 mountains have been elevated through large fissures, the 

 friction of the ascending masses against the sides of the 

 fissures has occasioned the basalt or the trachyte to be sur* 

 rounded by conglomerates, formed from fragments of their 

 own substance. ' In the sandstones of many formations, the 

 grains of which they are composed have been separated, 

 rather by the friction of erupted plutonic or volcanic rocks, 

 than by the erosive action of a neighbouring sea. The ex- 

 istence of this species of conglomerate (which is found in 

 immense masses in both continents), testifies the intensity of 

 the force with which the eruptive masses were impelled from 

 the interior towards the surface. The pulverized materials 

 must have been subsequently conveyed away by the waters, 

 and disseminated in the beds where they are now found." ( 296 ) 

 Formations of sandstone are found every where interposed 

 between other strata, from the lower silurian series to the 

 tertiary formations above the chalk. On the margins of 

 the vast plains of the new continent, both within and beyond 

 the tropics, we find these beds of sandstone extending in 

 long ramparts or walls, as if indicating the ancient shore 

 against which the billows 1 of the sea once broke. 



When we glance at the geographical distribution of rocks, 

 and at the extent which each occupies of the portion of the 

 crust of the earth accessible to our researches, we recognise 

 that the most generally prevailing chemical substance is 



