METAMORPHOSES OF ROCKS. 



77 



non, the vast significance of which, as proclaim- 

 ing the influence of organic activity upon the 

 formation of the sohd constituents of the earth, 

 was discovered in very recent times, by my in- 

 tellectually-gifted friend and fellow-traveller, 

 Ehrenberg. 



If in this short but comprehensive survey of 

 the mineral constituents of the crust of the 

 earth, we do not immediately refer to numbers 

 of simple sedimentary rocks, the various con- 

 glomerate and sandstone formations, partly de- 

 posited from liquids, that are so variously in- 

 termingled with the schists and the limestones 

 both of the floetz and transition series, this is 

 only because these, besides fragments of erup- 

 ted and sedimentary rocks, also contain pieces 

 of gneiss, mica-schist, and other metamorphic 

 masses. The obscure process of transforma- 

 tion (metamorphosis), and the influence it ex- 

 erts, must, from this showing, constitute the 

 third class of fundamental forms. 



The endogenous or eruptive rocks (granite, 

 porphyry, and melaphyre), exert an influence, 

 as already oftener than once observed, not 

 merely of a dynamical kind, shattering or up- 

 heaving, erecting or pushing strata aside ; by 

 their presence they farther produce changes in 

 the chemical composition of their constituents, 

 as well as in the nature of their intimate tex- 

 ture. Ne^ species of rocks are produced, 

 gneiss and mica slate, and granular or sac- 

 charoidal limestone (Carrara and Parian mar- 

 ble). The old Silurian or Devonian transition 

 schists, the belemnitic limestone of Tarantaise, 

 the grey unlustrous macigno or cretaceous 

 sandstone of the Northern Apennines, with its 

 included sea-weed, are difficult of recognition 

 after their transformation into new and fre- 

 quently-sparkling textures. The belief in the 

 metamorphosis, indeed, has only been confirm- 

 ed since we have succeeded in following the 

 several phases of the transformation, step by 

 step, and have come to the assistance of in- 

 ductive conclusions with the results of direct 

 chemical experiments, the employment of dif- 

 ferent fusing heats, degrees of pressure, and 

 rates of cooling. When the study of chemical 

 combinations is extended under the guidance 

 of leading ideas("8), we find that from the nar- 

 row confines of our laboratories, we can dif- 

 fuse a clear light over the wide field of geolo- 

 gy, over the great subterraneous rock-compo- 

 sing and rock-transforming workshop of Na- 

 ture. The philosophical incpirer escapes being 

 deceived by seeming analogies, by limited views 

 of the natural processes, when he keeps steadi- 

 ly in his eye the complication of circumstan- 

 ces which, in the intensity, the immeasurable- 

 ness of their force, were competent, in the 

 primitive world, to modify the reciprocal influ- 

 ences of individual substances familiarly known 

 to us at the present day. The simple or unde- 

 composed bodies have unquestionably obeyed 

 the same forces of affinity at all times ; and 

 where contradictions seem to meet us now, it 

 is my most intimate persuasion, that chemistry 

 will herself, for the most part, come upon the 

 traces of conditions not fulfilled in like or due 

 measure, as causes of these contradictions. 



Accurate observations, embracing extensive 

 districts of mountainous country, satisfy us that 



the eruptive rocks do not inteivene as any dig- 

 orderly or lawless power. In the most distant 

 countries of the world, we frequently see gran- 

 ite, basalt, or diorile, exerting their transform- 

 ative force, in every the most minute particu- 

 lar, alike upon strata of clay-slate, on thick 

 beds of limestone, and on the grains of quartz 

 of which sandstone consists. As the same 

 kind of endogenous rock almost everywhere 

 exerts the same kind of influence, diffisrent 

 kinds of rocks belonging to the same class ol 

 endogenous or eruptive formations, exhibit, on 

 the contrary, very different characters. In- 

 tense heat, above all, has exerted an influence 

 in the whole of the phenomena ; but the degree 

 of molten fluidity attained — perfect mobility of 

 particles, or a more viscid or glutinous adhesion 

 among them — has been very different in gran- 

 ite and basalt ; in different geological epochs, 

 indeed (phases in the transformation of the crust 

 of the earth), along with the eruptions of gran- 

 ite, basalt, porphyritic greenstone or serpentine, 

 various other substances dissolved in vapours 

 have arisen from the interior laid open. And 

 this is the place to remind the reader anew, 

 that in the rational views of modern geology, 

 the metamorphosis of rocks is not limited to the 

 mere phenomena of contact, to the apposition 

 of two different kinds of rock ; but that geneti- 

 cally it comprises all that has accompanied the 

 protrusion of a particular ejected mass. In 

 situations where no immediate contact has 

 taken place, the mere vicinity of such a mass 

 causes modifications in the induration, the sil- 

 icification, the granulation, the crystallization 

 of adjacent rocks. 



All eruptive rocks penetrate the sedimentary 

 strata, and other likewise endogenous masses, 

 as veins ; but the distinction that is apparent 

 between the Plutonic rocks("') — granite, por- 

 phyry, serpentine — and those which, in a more 

 restricted sense, are called volcanic (trachyte, 

 basalt, lava), is of especial importance. The 

 rocks which our present volcanoes, as rem- 

 nants of the activity of the body of the earth, 

 produce, appear in narrow streams, which, 

 however, may still form sufficiently wide beds 

 when several of them meet in hollows or ba- 

 sins. Basaltic eruptions, where they have been 

 traced deeply, have been repeatedly seen to 

 terminate in slender taps. Flowing from nar- 

 row openings, as in the Pftasterkaute, near 

 Marksuhl, two miles from Eisenach, in the blue 

 knolls, near Eschwega (banks of the Werra), 

 and at the Druid's-stone, on the Hollert ridge 

 (Siegen), to cite three examples indigenous to 

 Germany, the basalt breaks through the red 

 sand-stone and greywacke schist, and spreads 

 out above, like the cap of a mushroom, into 

 knolls, which in one place appear split into 

 columnar groups, in another are thinly strati- 

 fied. Not so granite, syenite, quartzose, por- 

 phyry, serpentine, and the entire series of un- 

 stratified massy rocks, which, from an attach- 

 ment to mythological nomenclature, have been 

 called Plutonic. These, with the exception of 

 a few veins, have been ejected, not in a molten 

 liquefied state, but in one merely tenacious and 

 softened, and not from narrow crevices, but 

 from wide valley-like chasms, and extensive 

 gorges. They have been forced, they have not 

 flowed out : they present themselves not in 



