ROOKS. 27! 



Conglomerates and rocks of detritus, when considered in the 

 widest sense of the term, manifest characters of a double 

 origin. The substances which enter into their mechanical 

 composition have not been alone accumulated by the action 

 of the waves of the sea, or currents of fresh water, for there 

 are some of these rocks the formation of which cannot be 

 attributed to the action of water. " When basaltic islands 

 and trachytic rocks rise on fissures, friction of the elevated 

 reck against the walls of the fissures causes the elevated rock 

 to be inclosed by conglomerates composed of its own matter. 

 The granules composing the sandstones of many formations 

 have been separated, rather by friction against the erupted 

 volcanic or plutonic rock, than destroyed by the erosive force 

 of a neighbouring sea. The existence of these friction con- 

 glomerates, which are met with in enormous masses in both 

 hemispheres, testifies the intensity of the force with which the 

 erupted rocks have been propelled from the interior through 

 the earth's crust. This detritus has subsequently been taken 

 up by the waters, which have then deposited it in the strata 

 which it still covers."* Sandstone formations are found im- 

 bedded in all strata, from the lower silurian transition stone 

 to the beds of the tertiary formations, superposed on the chalk. 

 They are found on the margin of the boundless plains of the 

 new continent, both within and without the tropics, extend- 

 ing like breastworks along the ancient shore, against which 

 the sea once broke in foaming waves. 



If we cast a glance on the geographical distribution of 

 rocks, and their relations in space, in that portion of the earth's 

 crust which is accessible to us, we shall find that the most 

 universally distributed chemical substance is silicic acid, 

 generally in a variously coloured and opaque form. Next to 

 solid silicic acid, we must reckon carbonate of lime, and then 

 the combinations of silicic acid with alumina, potash, and 

 soda, with lime, magnesia, and oxide of iron. 



The substances which we designate as rocks are determi- 

 nate associations of a small number of minerals, in which 

 some combine parasitically, as it were, with others, but only 



Leop. von Buch, Geogwst. Briefe, s. 75-82, where it is also shown 

 why the new red sandstone (the Todtliegende of the Thuringian Flb'ta 

 formation), and the coal measures, must be regarded as produced by 

 erupted porphyry. 



