ROCKS. 269 



potash not excepted * It would not b(3 very surprising, there* 

 fore, as is well observed by the distinguished geognosist, Von 

 Dechen, if we were to meet with a fragment of gneiss formed 

 on the walls of a smelting furnace which was built of argilla- 

 ceous slate arj^ graywacke. 



After having taken this general view of the three classes 

 of erupted, sedimentary, and metamorphic rocks of the earth's 

 crust, it still remains for us to consider the fourth class, com- 

 prising conglomerates, or rocks of detritus. The very term 

 recalls the destructi&a which the earth's crust has suffered, 

 and likewise, perhap*^ reminds u? of the process of cementation, 

 which has connected together, by means of oxyd of iron, or of 

 some argillaceous and calcareous substances, the sometimes 

 rounded and sometimes angular portions of fragments. Con- 

 glomerates 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 can not be attributed to the action of 

 water. " When basaltic islands and trachytic rocks rise on 

 fissures, friction of the elevated rock against the walls of the 

 fissures causes the elevated rock to be inclosed by conglom- 

 erates 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 neighboring sea. The ex- 

 istence of these friction conglomerates, 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. "t Sand- 

 stone formations are found imbedded in all strata, from the 

 lower Silurian transition stone to the beds of the tertiary form- 

 ations, superposed on the chalk. They are found on the 

 margin of the boundless plains of the New Continent, both 

 within and without the tropics, extending like breast-worka 

 along the ancient shore, against which the sea once broke in 

 foaming waves. 



* D'Aubuisson, ia the Journal de Physique, t. Ixviii., p. 128. 



T Leop. von Buch, Geognost. Brief e, s. 75-82, where it is also shown 

 why the new red sandstone (the Todtliegende of the Thuringian flSta 

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

 erupted porphyry. 



