ROCKS. 249 



ther by contact or proximity with a Plutonic or volcanic en- 

 dogenous rock of eruption,* or, what is more frequently the 

 case, by a gaseous sublimation of substancest which accom- 

 pany certain masses erupted in a hot, fluid 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 or 

 chemical precipitation from liquids impregnated v/ith 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 wliich must have characterized 

 the chaotic condition of the earlier world under wholly difier- 

 ent conditions of pressure and at a higher temperature, not 

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



* lu a plan of the neighborhood of Tezcuco, Totouilco, and Moran 

 (Atlas Giographique et Physique, pi. vii.), which I originally (1803) 

 intended for a woi'k which I never published, entitled Pasigrafia Geog- 

 nostica destinada al uso de los Jovenes del Colegio de Mineria de Mexi- 

 co, I named (in 1832) the Plutonic and volcanic ex'uptive rocks endoge- 

 nous (generated in the interior), and the sedimentary and flotz rocks 

 exogenous (or generated externally on the surface of the earth). Pasi- 

 graphically, the former w^ere designated by an arrow directed up- 

 ward f , and the latter by the same symbol directed downward |. 

 These signs have at least some advantage over the ascending lines, 

 which in the older systems represent arbitrarily and ungracefully the 

 horizontally ranged sedimentary strata, and their penetration through 

 masses of basalt, porphyry, and syenite. The names proposed in the 

 pasigraphico-geognostic plan were borrowed from De Candolle's nomen 

 clature, in which endogenous is synonymous with monocotyledonous, 

 and exogenotis with dicotyledonous plants. Mohl's more accurate ex- 

 amination of vegetable tissues has, however, shown that the growth of 

 monocotyledons from w^ithin, and dicotyledons from without, is not 

 strictly and generally true for vegetable orga^iisms (Link, Elementa 

 Philosophice Botanicce, t. i., 1837, p. 287; Endlicher and Unger, Grund- 

 zuge der Botanik, 1843, s. 89 ; and Jussieu, TraiU de Botanique, t. i., 

 p. 85). The rocks which I have termed endogenous are characteristic- 

 ally distinguished by Lyell, in his Principles of Geology, 1833, vol. iii., 

 p. 374, as " nether-formed" or " hypogene rocks." 



t Compare Leop. von Buch, Ueber Dolom.it als Gehirgsart, !t823, s. 

 36 ; and his remarks on the degree of fluidity tp be ascribed to Plutonic 

 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 Akad. der Wissen- 

 »ch. zu Berlin for the year 1842, s. 58 und 63, and in the Jahrhucli fui 

 Wissenschaftliche Kritik, 1840, s. 195. 



L 2 



