n^ 



76 



FUNDAMENTAL FORMS OF ROCKS. 



sent themselves to us as large embedded crys- 

 tals. In granitic porphyry, which is very poor 

 in quartz, the felspathic base is at once granu- 

 lar and foliaceous(*=**). 



Greenstone or Diorite — granular mixtures 

 of white albite and blackish-green hornblende, 

 constituting dioritic porphyry, when a base of 

 denser texture is present in which the crystals 

 lie embedded distinctly. These greenstones, 

 which, pure in one place, pass in another into 

 serpentine, from the laminae of diallage which 

 they include (Fichtelgebirge), are occasionally 

 found lying in beds upon the old stratification 

 clefts of the green clay slate, and penetrating 

 them ; but they more frequently make their 

 way through the rock in the manner of veins, 

 or they present themselves as greenstone balls, 

 analogous in all respects to balls of basalt and 

 porphyry("^). 



HypERSTHENE RocK — a granular mixture of 

 Labrador felspar and hypersthene. 



EuPHOTiDE and Serpentine, occasionally con- 

 taining crystals of augite and uralite, instead of 

 diallage, and thus nearly allied to a more com- 

 mon rock, and, I might add, one that indicates 

 a still higher degree of eruptive activity,' viz., 

 augitic porphyry(^2®). 



Melaphyre, Augitic, Uralitic, and Oligo- 

 GLAssic Porphyry. To the last belongs the true 

 verd antique, so celebrated as a material em- 

 ployed in the arts. 



Basalt, with olivine and constituents becom- 

 ing gelatinous with acids, phonolite (porphy- 

 ritic slate), trachyte and dolerite. The sec- 

 ond of these rocks always divides into thin ta- 

 bles ; the first only shows this structure par- 

 tially, which, however, gives them both an ap- 

 pearance of stratification over extensive dis- 

 tricts. According to Girard,mesotype and neph- 

 eline form important elements in the composi- 

 tion and intimate texture of basalt. The neph- 

 eline of basalt reminds the geologist of the mi- 

 ascite of the Ilmengebirge in the Ural chain("''), 

 which frequently replaces granite, and occasion- 

 ally contains zircon, as well as of the pyroxenic 

 nepheline discovered by Gumprecht near Lobau 

 and Chemnitz. 



To the second class of fundamental forms, 

 the sedimentary rocks, belongs the greater 

 portion of the formations which used to be ar- 

 ranged under the old systematic, but by no 

 means correct, designation of Transition and 

 Floetz, or secondary and tertiary formations. 

 Had the igneous rocks exerted nothing of an 

 uplifting, and, with simultaneous quaking of the 

 earth, of a concussive influence upon these 

 sedimentary formations, the surface of our 

 planet would have consisted of a series of uni- 

 form strata horizontally disposed one upon an- 

 other. Without mountains, on whose acclivi- 

 ties the progressive diminution in the tempera- 

 ture of the air is picturesquely reflected, not 

 only in the luxuriance of vegetation, but in the 

 kinds of plants that are produced, the monot- 

 onous surface would only have been broken 

 here and there by ravines eroded by water- 

 courses or by small collections of drift, the ef- 

 fect of masses of fresh water thrown into gen- 

 tle undulations ; the several continents from 

 pole to pole, and under every variety of cli- 

 mate, would have presented the dreary uni- 



formity of the South American Llanos or of the 

 Northern Asiatic steppes. As in the greater 

 ■ portion of these, we should then have seen the 

 vault of heaven resting on the plain, and the 

 stars rising and setting as if they emerged 

 from the bosom of the ocean, and dipped into it 

 I again. But such a state of things even in the 

 primitive world could never have been of any 

 i considerable duration as regards time, nor of 

 ! any thing like general prevalence in respect of 

 space ; the subterraneous powers, at every 

 epoch in the history of nature, have been at 

 \ work striving to subvert and to change it. 

 I Sedimentary strata are precipitated or de- 

 j posited from liquids, according as the matter 

 j before the formation was either held chemical- 

 i ly dissolved, as in the case of lime, or merely 

 I suspended and mixed, as in the case of clay- 

 I slate, mica-slate, &c. But even when earthy 

 I matters are thrown down from fluids impreg- 

 j nated with carbonic acid, the descent of the 

 matter during its precipitation and accumula- 

 I tion into strata, must be regarded as a me- 

 I chanical element in the process of formation. 

 This view is of some importance in connection 

 with the envelopment of organic bodies in pet- 

 rifying calcareous tuffs. The oldest sediments 

 of the transition and secondary formations 

 have apparently taken place from waters more 

 or less elevated in temperature, and at a pe- 

 riod when the heat of the upper crust of the 

 earth was still very considerable. In this way, 

 therefore, a Plutonic influence was also at work 

 to a certain extent in connection with the sedi- 

 mentary strata, particularly the oldest of them ; 

 these strata, however, appear to have become 

 hardened from the state of mud into the schis- 

 tose structure, under great pressure ; not like 

 the rocks that have risen up from the interior 

 (granite, porphyry, basalt), to have been con- 

 solidated by cooling. As the primitive waters 

 of the globe cooled by degrees, they became 

 capable of holding a larger and larger quantity 

 of carbonic acid gas in solution, which they 

 may have attracted from the atmosphere, sur- 

 charged with this gas in the earlier epochs of 

 creation, and so of holding dissolved a larger 

 quantity of calcareous earth. 



The Sedimentary strata, from which we 

 here separate all the other exogenous purely 

 mechanical precipitates of sand or fragmentary 

 rocks, are these ; 



Schists or Slates of the inferior and supe- 

 rior transition rocks, consisting of the Silurian 

 and Devonian formations ; from the lower Si- 

 lurian,, or as they were once designated, Cam- 

 brian, strata, to the uppermost bed of the Old 

 red sandstone or Devonian formation, where it 

 comes in contact with the Mountain limestone ; 



Carboniferous deposits — Coal formation ; 

 . Limestones, interstratified in the transition 

 and coal formations ; Zechstein, Muschelkalk, 

 Jura formation and Chalk, also the portion of 

 the tertiary formation which does not present 

 itself to us as sandstone and conglomerate ; 



Travertine, fresh-water limestone, the si- 

 licious sinter of hot springs — formations .that 

 have originated not under the pressure of great 

 pelagic coverings of water, but almost in con- 

 tact with the air in shallow pools and rivulets ; 



Infusorial strata, a geological phenome* 



