ROCKS 249 



oose, by a gaseous sublimation of substances'* which accom- 

 pany certain masses erupted in a hot tiiiid condition. 



Conglomerates ; coarse or finely granular sandstones, or 

 breccias composed of mechanically divided masses of the three 

 previous species. 



These four modes of formation, by the emission of volcanic 

 masses, as narrow lava-streams ; by the action of these masses 

 on rocks previously hardened ; by mechanical separation cr 

 chemical precipitation from liquids impregnated with carbonic 

 acid ; and finally, by the cementation of disintegrated rocks 

 of heterogeneous nature are phenomena and formative pro- 

 cesses, which must merely be regarded as a faint reflection of 

 that more energetic activity which must have characterised the 

 chaotic condition of the earlier world, under wholly different 

 conditions of pressure, and at a higher temperature, not only 

 in the whole crust of the earth, but likewise in the more 

 extended atmosphere, overloaded with vapours. The vast 

 fissures which were formerly open in the solid crust of the 

 earth have since been filled up or closed by the protrusion of 

 elevated mountain chains, o'r by the penetration of veins of 

 rocks of eruption (granite, porphyry, basalt, and melaphyre) ; 

 and whilst on a superficial area equal to that of Europe there 

 are now scarcely more than four volcanoes remaining, through 

 which fire and stones are erupted, the thinner, more fissured, 

 and unstable crust of the earth was anciently almost every - 



phieo-geognostic plan were borrowed from Decandolle's nomenclature, 

 in which endogenous is synonymous with monocotyledonous, and exo- 

 genous with dicotyledonous, plants. Mohl's more accurate examination 

 of vegetable tissues has, however, shown that the growth of monocotyle- 

 dons from within, and dicotyledons from without, is not strictly and 

 generally true for vegetable organisms (Link, Elementa PkilwopMce 

 BotaniccK, i. i. 1837, p. 287 ; Endlicher and linger, Grundzuge der 

 Botanik, 1843, s. 89 ; and Jussieu, Trait6 de Botanique, t. i. p. 85.) 

 The rocks which I have termed endogenous are characteristically distin- 

 guished by Lyell, in his Principles of Geology, 1833, vol. iii.* p. 374, 

 as " nether-formed" or " hypogene rocks." 



* Compare Leop. von Buch, Ueber Dolomit a2s Gebirgsart, 1823, 

 6. 36 ; and his remarks on the degree of fluidity to be ascribed to plu- 

 tonic rocks at the period of their eruption ; as well as on the formation 

 of gneiss from schist, through the action of granite and of the substances 

 upheaved with it ; to be found in the Abhandl der A had. der Wissen- 

 ich. zu Berlin, for the year 1842, s. 58 und 63, and in the JahrbuchfM 

 Kritik, 1840, a. 195. 



