78 



METAMORPHOSES OF ROCKS. 



streams, like lava, but spread out in immense 

 massesC"). Among the dolerites and tra- 

 chytes, some groups give indications of a cer- 

 tain basalt-like fluidity ; others, expanded into 

 vast bell -shaped elevations and craterless 

 domes, appear to have been merely softened 

 when they were protruded. Other trachytes, 

 again, those of the Andes among the number, 

 which I frequently found very closely allied to 

 the greenstones and syenitic-porphyries, so rich 

 in silver, and then without quartz, lie in beds 

 like granite and quartzose-porphyry. 



Experiments upon the alterations which the 

 structure and chemical constitution of rocks 

 undergo through lire("^), have showed that the 

 volcanic masses, diorite, augitic porphyry, ba- 

 salt, and lava from JStna, according to the de- 

 gree of pressure under which they were melted, 

 and the rate of their cooling, were either, when 

 quickly cooled, brought to the state of a black 

 glass of an even fracture, or when slowly cool- 

 ed made to assume the appearance of a stony 

 mass having a granular crystalline texture. 

 The crystals in such cases were either pro- 

 duced on the sides and cavities, or embedded 

 in the general basic mass. The same material 

 — and this consideration is of great importance 

 as regards the nature of the eruptive rock, or 

 the transformations it has undergone — yields 

 the most dissimilar-looking products. Carbo- 

 nate of lime, melted under high pressure, does 

 not lose its charge of carbonic acid ; the cooled 

 mass is granular limestone, saccharoidal mar- 

 ble. So much for crystallization in the dry 

 way ; in the moist way, calcareous spar as well 

 as Aragonite is produced, the former under a 

 moderate, the latter under a higher, degree of 

 heatC^'"). According to diversities of temper- 

 ature, the consolidating particles of crystals in 

 process of formation arrange themselves va- 

 riously and in particular determinate direc- 

 tions ; the very form of the crystals, indeed, 

 varies with the temperature under the influ- 

 ence of which they are produced(=^"). There 

 is, moreover, under certain relations, and with- 

 out the intervention of any fluid state, a trans- 

 positions^^*) of the minute particles of a body, 

 which is proclaimed by optical effects. The 

 phenomena presented by devitrifaction, by the 

 production of cemented and cast steel, by the 

 transition of the fibrous structure of iron into 

 one that is granular, under the influence of el- 

 evated temperature(^^*), perhaps even of very 

 insignificant but equable and long-continued 

 concussions, all conduce to throw light upon 

 the processes of geological metamorphosis. 

 Heat can even induce opposite effects at the 

 same time upon crystalline bodies ; for Mits- 

 cherlich's beautiful experiments show that cal- 

 careous spar, without altering its state of ag- 

 gregation, expands in the direction of one of 

 its axis of crystallization, and contracts in an- 

 other("«). 



If from these general considerations we pass 

 on to particular examples, we first observe 

 schists turned into black-blue roofing slate by 

 the vicinity of Plutonic ejected rocks. The 

 clefts of stratification are then interrupted by 

 another system of clefts which cut the former 

 almost perpendicularly, and indicate the opera- 

 tion of a later influence("'). By the penetra- 

 tion of silicic acid, clay slate, traversed by frag- 



! ments of quartz, is partially changed into whet- 

 I stone slate (Wetzschiefer, whitestone or Eu- 

 i ritel) and silicious slate (Kieselschiefer, quart- 

 I zitel), the latter frequently carboniferous, and 

 i then galvanic in its effects on the nerves. The 

 j highest degree of silicification of the schists("«), 

 however, is found in a precious material em- 

 ployed in the arts, ribboned jasper, produced 

 in the Ural Mountains by the contact of erup- 

 tive augitic porphyry (Orsk), dioritic porphyry 

 (Auschkul), or hypersthene rock (Bogoslowsk) ; 

 in the Island of Elba (Monte Serrato), accord- 

 ing to Fr. Hoffmann, and in Tuscany, accord- 

 ing to Alexander Brongniart, by contact with 

 euphotide and serpentine. 



The contact and Plutonic influence of granite 

 cause clay-slate to become granular, changing 

 it into a granitic-looking mass — into a mixture 

 of felspar and mica, in which again larger plates 

 of mica lie embedded("9) — a fact which Gus- 

 tavus Rose and I observed within the fortress 

 of Buchtarminsk(2«). «'That the whole of 

 the gneiss lying between the Icy Sea and the 

 Gulph of Finland has been formed and trans- 

 formed by the agency of granite out of Silurian 

 strata of the transition series, may now, as 

 Leopold von Buch has said, be assumed as an 

 hypothesis familiar to all geologists, and ac- 

 cepted by the greater number as demonstrated. 

 In the Alps of St. Gothard cretaceous marl is 

 met with transformed by granite, ffrst into mi- 

 caceous schist, and then into gneiss"(^"). 

 Similar phenomena in respect of gneiss and 

 mica slate formations, under the influence of 

 granite, are presented : in the Oolitic group of 

 Tarantaise(=^*2), where belemnites have been 

 found in rocks that already lay claim to the de- 

 nomination of mica schist ; in the schistose 

 group of the western portion of the Island of 

 Elba, not far from Cape Calamata, and in the 

 Fichtelgebirge of Bayreuth, between Lomitz 

 and Markleiten(^"). 



Precisely as jaspar, a substance employed in 

 the arts, which was inaccessible to the ancients 

 in large masses(2**), is the product of volcanic 

 agency upon augitic porphyry, so is the other 

 artistic material, so variously and so success- 

 fully employed by them, granular marble, to be 

 regarded as a sedimentary stratum altered by 

 the heat of the earth and the vicinity of an 

 eruptive rock. Careful observation of phe- 

 nomena of contact, and the remarkable experi- 

 ments of Sir James Hall on the fusion of rocks, 

 now more than half a century old, in addition 

 to the diligent study of granitic veins, which 

 contributed so essentially to the early founda- 

 tions of our present geology, warrant such a 

 conclusion. The protruded rock has occasion- 

 ally changed the dense calcareous deposit into 

 granular limestone to a certain thickness only, 

 or in a certain zone from the line of contact. 

 We find a partial transformation, like a half- 

 shadow, at Belfast in Ireland, where basaltic 

 dykes penetrate the chalk ; in the same way, 

 in the compact floetz-limestone near the bridge 

 of Boscampo, and by the waterfall of Canzocoli 

 in the Tyrol,, celebrated by Count Marzari Pen- 

 cati, the strata have been partially bent where 

 they come in contact with a syenitic granite(=^**). 

 Another kind of transformation is that in which 

 the whole of the beds of compact calcareous 

 rock are changed into granular limestone 



