April i, 1880] 



NATURE 



529 



those of Africa, although this continent is so near to Europe, 

 and its mammalian fauna at the diluvial period was in intimate 

 connection with that of Sonthern Europe. 



ON THE ORIGIN OF THE MINERAL, STRUC- 

 TURAL, AND CHEMICAL CHARACTERS OF 

 OPHITES AND RELATED ROCKS 1 

 HPHE authors, beginning with (A) "The different kinds of 

 •^ rocks treated of," in their memoir, divide them into two 

 groups. The first, "Silacid Ophites," is represented by ser- 

 pentinite (common at the Lizard) and other rocks, essentially 

 composed of serpentinous minerals : it includes a sub-section, 

 comprising peridolites and some others, all slightly hydrated. 

 The second, "Silocarbacid Ophites," consists of rocks, which, 

 in addition to serpentinous minerals, contain a mineral carbonate 

 — for example, ophi-calcite ; its sub-section is represented by 

 hemithrenes. The relation of the first group, through its sub- 

 section, to ordinary metamorphic rocks, also of the second 

 group, through its subsection, to carrarites and dolomites is 

 pointed out. 



As regards (B) " Their mineral character," it is stated that 

 ophites, &c, embrace some fifty or more different minerals, all 

 containing more or less hydrous silicate of magnesia, in addition 

 to which dry silicates and carbonates are often present. The 

 relation of these minerals to others, essentially anhydrous, as 

 hornblende, diallage, and peridote, is noticed. 



Treating on (C) " The structural character of ophites, &c." 

 the protean nature of their essential mineral, serpentine, is shown 

 by a description of its fibrous, arborescent, coccolitic, platy, and 

 other allomorphs. 



With reference to (D) " The origin of certain mineral, struc- 

 tural, and chemical characters of ophites, &c," the subject is 

 treated of under different heads : — 1. Fibrous layers in peridote 

 from Elfdalen, in " graphic granite " from Harris, in perthite 

 from Siberia, and in other instances. 2. The alternation of dif- 

 ferent minerals in laminated ophicalcite and ophimalacolite has 

 its parallel in other and totally different rocks. 3. The change 

 of the fibres of chrysotile into aciculce, separated by calcareous 

 interpolations, is illustrated by figures taken from decalcified and 

 polarised specimens of this allomorph from Canada ; also from 

 a characterise specimen of the same from the type-locality; 

 Reichenstein. 4. Branching configurations, such as are assumed 

 by serpentine, are common in hetuithrenes from widely different 

 regions ; the authors refer to examples, showing that they are 

 residual, resulting from the waste of crystalloids of rnalacolile. 

 Beautiful examples occur in the calcaire saccharoide (a henii- 

 threne) of St. Philippe, near St. Marie-aux-Mii.es, in the Vosges, 

 rivalling those in Canadian ophite ; and not only are the 

 associated lobulated grains of pyrosclerite covered with a fibrous 

 layer, closely resembling chrysotile in structure ; but its fibres 

 are occasionally converted into aciculre, separated by films of 

 calcite. 5. The presence of calcite under the latter condition, 

 and in connection with configurations of serpentine and malaco- 

 lite, as well as lobulated grains of these and other minerals, the 

 authors ascribe to chemical changes similar to pseudomorphism 

 among minerals. 6. It is contended that no minerals are in- 

 capable of resisting changes of the kind, even those regarded as 

 the most insoluble. The experiments of Bischof, the Professors 

 W. B. and K. E. Rogers and others, show that hypersthene, 

 enstatite, serpentine, and various mineral silicates, digested in 

 water containing carbonic acid, are convertible into carbonates. 

 7. Cases are mentioned of rocks, essen ially composed of mineral 

 silicates, which have thus become changed ; as diorite from 

 Jersey, granite and a porphyritic feld-ite from near Gahvay, 

 which have had certain of their mineral silicates replaced by ser- 

 pentine and calcite. 



The latter cases bring on a chapter (E) "On rock-meta- 

 morphism generally." The authors divide metamorphic rocks 

 into two groups — mineralised, and methylosed ; the former con- 

 sisting of members which have had their original sedimentary 

 components mineralised into gneiss, hornblende-schist, Sec ; and 

 the latter of members thus mineralised, but which, through the 

 intervention of chemical reactions, have been converted into 

 ophites, &c. ; methylosisis to rocks the same as pseudomorphism 

 to minerals. After briefly referring to the theory promulgated 

 by Leibnitz in his "Protogoe," which anticipated many points 

 now generally held as to the origin of the metamorphics, they 



■ Paper read at the Royal Society by Professors W. King,' Sc.D., and T. 

 H. Rowney, Ph.D.- 



examine the doctrine advocated by Sterry Hunt ; and contend 

 that it is altogether untenable, both from his own arguments, 

 and a body of unquestionable counter-evidence. Repudiating a 

 doctrine which regards the rocks in question as still being in 

 their original or quasi-original condition, formed at the bottom 

 of a primaeval ocean, through the chemical precipitation of 

 substances which it held in solution, the authors express them- 

 selves in accordance with the prevailing opinion that they were 

 originally ordinary argillaceous, arenaceous, and other sediments, 

 which, through being buried at great depths, have undergone 

 various changes — some ending in their mineralisation, and others 

 in tbeir methylosis. Sterry Hunt's doctrine is further contested 

 by evidences adduced of regional metamorphosism pertaining to 

 various post-Archaean periods, whose crystalline or mineral 

 effects are identical with those which he restricts to pre-Cambrian 

 ages, and which he presumes to have been produced by chemical 

 precipitations from seas of the time. 



The mineralised metamorphics having thus far principally 

 engaged their attention, the authors next touch upon the,(F) 

 " Methylosed metamorphics — ophites." Taking, as their stand- 

 point, the carefully worked out conclusion of Bliim, Bischof, 

 Rose, and others, that serpentine, as a mineral, is in all cases 

 the product of pseudomorphism, it is contended that rocks 

 essentially made up of it, adding other secondary minerals in 

 certain kinds, have necessarily undergone chemical changes. 

 Cases are cited, such as the serpentinite of the lizard, which they 

 were the first to show, from its containing pseudomorph crystals 

 after augite, had been originally a porphyritic dolerite. One of 

 the Cannover Isles, in Lough Corrib, contains a mass of ser- 

 pentine, which is shown to be a methylosed diorite or tremolitic 

 rock. 



The evidences offered by Bischof, Heddle, and other writers, 

 as to the conversion of serpentinous and other siliceous rocks 

 into calcareous masses are adduced by the authors in confirmation 

 of their view respecting (G) "The methylotic origin of hemi- 

 threnes, &c." Additional original evidences are brought forward 

 with the same purport. A volcanic or doleritic dyke intersects 

 gneiss on Mr. Frederick Twining's estate, adjacent to Cleggan 

 Biy, Connemara ; where, not only is the gneiss converted into 

 hemithiene, consisting of malacolite, peridote, serpentine, calcite, 

 and other minerals, but the dyke itself is charged with calcific 

 matter. Another case occurs at St. Philippe, Vosges, where 

 gneiss incloses dyke-like masses of hemithrene, as to conclusively 

 prove, in the opinion of the authors, that the latter are chemically 

 changed products of the former, effected by permeating streams 

 of heated water containing a carbonate in solution. The labours 

 of I >elesse have shown that the region around abounds with 

 masses of the kind. 



The rocks described having undergone such' remarkable 

 changes, the authors have been induced to make investigations 

 as to (H) "The origin of the minerals characteristic of ophites, 

 &c, especially peridote." With certain exceptions the minerals 

 referred to are considered to be of secondary origin, the excep- 

 tions being those remaining unaffected by secondary agencies. 

 Serpentines, malacolite, phlogopite, chlorite, enstatite, and a 

 number of others are all considered as secondary minerals. 

 Peridote, notwithstanding that it is generally considered to be an 

 original mineral in the same sense as the hornblende, feldspar, 

 mica, &c, of granite and other plutonic rocks, is regarded by 

 the authors as a product of alteration in all its relations, and 

 circumstances of occurrence. Its presence in granites, basalts, 

 and lavas has given rise to the belief that it is of igneous origin : 

 nevertheless, its occurrence in mineralised and methylosed rocks 

 (gneiss, and ophite of the sedimentary section) is held as proving 

 the contrary; and the authors feel themselves justified in 

 assuming that it is as much a secondary product as the zeolites 

 and pseudomorphs found in granites, basalts, and lavas. Many 

 of the crystals occurring in ba-alts and lavas, which have been 

 taken for peridote, are in their opinion pseudomorphs after 

 augite and hornblende. 



Repudiating the doctrine that the Archaian rocks are the 

 result of chemical precipitations, and entertaining the strongest 

 doubts that life has been to any extent concerned in their forma- 

 tion, the authors, in a chapter (I) " On the origin of the Archaean 

 crystallite limestones of Canada," apply their views on hemi- 

 threnes to the present subject ; and they arrive at the conclusion, 

 from van us considerations, that the rocks in question are 

 methylosed products ; but which, before this change took place, 

 existed as gneisses, hornblende-schists, and other mineralised 

 silacid metamorphics. 



The question (J) " Why limestones are so rare in formations 



