Sept. 27, 1888] 



NATURE 



521 



II. Nor ian. — The terrane thus designated by the author in 

 1870 is composed in great part of those stratiform rocks having 

 a base of anorthic feldspars, to which has been given the name 

 of norite. This terrane, however, includes intercalated strata of 

 gneiss, of quartzite, and of crystalline limestone, all of which 

 resemble closely those of the Upper Laurentian. These norite 

 rocks, which are sometimes called gabbros, are not to be con- 

 founded with the very distinct gabbros of the Huronian terrane. 

 nor yet with certain plutonic rocks having with them certain 

 mineralogical resemblances. The facies of the norites serves to 

 distinguish them. 



III. Arvonian. — This terrane is composed in great part of 

 petrosiliceous rocks, which pass into quartziferous porphyries. 

 With them, however, are intercalated certain hornblendic rocks, 

 sericitic schists, quartzites, oxides of iron, and, more rarely, 

 crystalline limestone. Th s terrane, indicated for the first time 

 as distinct by Dr. Henry Hicks, in Wales, in 1878, and named 

 by him, is regarded by Mr. Charles Hitchcock as constituting 

 in North America the lower portion of the Huronian. 



IV. Huronian. — This name was given by the author in 

 1855 to a terrane already recognized in North America, where 

 it rests in discordant stratification either upon the Laurentian 

 gneiss or upon the Arvonian petrosilex. It includes, besides 

 quaitzose, epidotic, chloritic, and calcareous schists, masses of 

 serpentine, and of lherzolite, together with euphotides, which 

 represent heroin the norites of the Norian terrane, often con- 

 founded with them under the common name of gabbro. This 

 Huronian terrane is greatly developed in the Alps, where it 

 constitutes the series of the greenstones ox pietre verdi. 



V. Montalban. — The studies of von Hauer in the Eastern 

 Alps, published in 1868, and those of Gerlach on the Western 

 Alps, published in the year following, agree in recognizing in 

 these regions two gneissic terranes — namely, an older or ancient 

 central gneiss, and a younger or recent gneiss ; this last, which is 

 petrographically very distinct from the old gneiss, being accom- 

 panied by micaceous and hornblendic schists. The studies of 

 Gastaldi, published in 1871, and those of Neri, in 1874, while 

 confirming the results of von Hauer and of Gerlach, furnish us 

 with further details respecting these terranes and their litho- 

 logical characters. It should here be remarked that all of these 

 observers appear to agree in placing the horizon of the pietre 

 verdi (Huronian) between the ancient gneiss (Laurentian) and 

 the recent gneiss. 



Before becoming acquainted with the first results of these 

 observers, the writer, from his own studies in North America, 

 was led to precisely similar conclusions, and in 1870 announced 

 the existence of a series of younger gneisses very distinct from 

 the old Laurentian gneisses, and accompanied by crystalline 

 limestones and by micaceous and hornblendic schists. To this 

 younger terrane, on account of its great development in the 

 White Mountains of New Hampshire, he gave in 1871 the 

 name of Montalban. This series appears to be identical with 

 the younger gneiss of the Alps ; the so-called Hercynian gneisses 

 and mica schists of Bavaria ; the granulites, with dichroite- 

 gneiss, mica-schists, and lherzolite of the Erzgebirge in Saxony ; 

 and similar rocks in the Scottish Highlands. The Mont- 

 alban terrane in North America contains not only crystalline 

 limestones, but beds of lherzolite and of serpentine, resembling 

 in this respect the Huronian and the Laurentian. It is in this 

 series, in North America at least, that are found the chief part 

 of the veins or endogenous masses of granite, which carry 

 beryl, tourmaline, and the ores of tin, of uranium, of tantalum, 

 and of niobium. 



Gastaldi, in an essay published in 1874,, declares that "the 

 pietre verdi properly so called" is found between " the ancient 

 porphyroid and fundamental gneiss " and "the recent gneiss, 

 which latter is finer-grained and more quartzose than the other." 

 This younger gneiss he also describes as a gneissic mica-schist, 

 and as a very micaceous gneiss passing into mica-schist, and often 

 hornblendic; the two gneissic series being, according to him, 

 easily distinguished the one from the other. To these two divi- 

 sions, superior to the ancient gneiss — that is to say, the tme 

 pietre verdi and the younger gneiss — Gastaldi adds a third division, 

 still more recent. This highest division contains considerable 

 masses of strata called by him argillaceous schists, and other- 

 wise lustrous, lalcose, micaceous, and sericitic schists. Asso- 

 ciated with these are also found quartzites, statuary and 

 cipolin marbles, with dolomite, karstenite, and sometimes horn- 

 blendic rocks and serpentines, the presence of which in this 

 division, and also among the recent gneisses, as well as "in the 



pietre verdi proper," was regarded by Gastaldi as justifying the 

 name of "the pietre verdi zone," often given by him to the 

 whole of this tripie group of crystalline schists, which he recog- 

 nized as younger than the central gneiss. 1 



VI. Taconian. — This third division, to which Gastaldi did 

 not give a distinctive name, has, as is well known, a very in- 

 teresting history in Italian geology. A terrane having the same 

 horizon and the same mineralogical characters is found developed 

 on a grand scale in North America, where it includes quartzites, 

 often schistose, and sometimes flexible and elastic, with crystal- 

 line limestones yielding both statuary and cipolin marbles. It 

 also contains deposits of magnetite and of hematite, as well as 

 important masses of limonite, which is epigenic in some cases of 

 pyrites, and in others of chalybite, two species which form, by 

 themselves, large masses in the undecayed strata. This same 

 terrane contains, moreover, roofing-slates, as well as lustrous 

 unctuous schists, ordinarily holding damourite, sericite, or pyro- 

 phyllite, but including, occasionally, chlorite, steatite, and horn- 

 blendic rocks with serpentine and ophicalcite. We also find 

 among these schists, which are met with at several horizons in 

 the terrane, layers which are visibly feldspathic, with others of 

 ill-defined character, which, however, are converted into kaolin 

 by sub-aerial decay. These same schists furnish remarkable 

 crystals of rutile, and also tourmaline, cyanite, staurolite, garnet, 

 and pyroxene. This terrane, which, moreover, appears to be 

 diamond-bearing, was described in 1859 by the late Oscar 

 Lieber, under the name of the Itacolumitic group. Eaton 

 already, in 1832, had pi iced the quartzites and the limestones, 

 which form the lower members of this group, in the Primitive 

 division. The argillites in the upper part of the group were 

 regarded as the inferior member of his Transition division, and 

 were, according to him, overlain unconformably by the fossili- 

 ferous graywacke (First Graywacke), made the upper member of 

 this same Transition division. In 1842, Ebenezer Emmons in- 

 cluded in what he then named the 'Laconic system the whole 

 of this crystalline series, to which he added the graywacke ; but 

 in 1844 he separated this latter, in which he had meanwhile 

 found a trilobitic fauna, and gave it the name of Upper Taconic ; 

 the inferior and crystalline portions being the Lower Taconic. 

 Many years of study have shown me that this upper division is 

 entirely independent of the Lower Taconic, with which the 

 fossil iferous graywacke series is found in contact only in certain 

 localities, while in many others it rests directly upon more 

 ancient crystalline terranes. Seeing, morever, that the Lower 

 Taconic is found without this graywacke, in a great number of 

 localities, from the Gulf of St. Lawrence as far as Alabama 

 to the south, and as far as Lake Superior to the west ; and 

 recognizing also the fact that the Upper Taconic is really a 

 part of the Cambrian (as was avowed by Emmons himself in 

 i860), the author proposed in 1878 to limit the use of the term 

 Taconic to the crystalline infra-Cambrian series which forms the 

 Lower Taconic of Emmons and the Itacolumitic group of Lieber, 

 and to call it the Taconian terrane. 



The history of the various attempts made by the partisans of 

 the metamorphic school to establish a more recent origin for the 

 Taconian is a curious one. Various American geologists, adopt- 

 ing for the most part stratigraphical arguments, have successively 

 referred it to the Cambrian, Ordovician, Silurian, Carboniferous, 

 and Triassic horizons. It is, however, to be noted that these 

 same geologists have also maintained the Palaeozoic age of the 

 greater part of the other crystalline terranes of North America, 

 comprising the Montalban, the Huronian, the Arvonian, and a 

 part of the Laurentian itself. The want of any conception of the 

 principle of mineralogical development in the history of the 

 crystalline schists, conjoined with the difficulties arising from the 

 stratigraphical complications met with at many points along the 

 eastern border of the great North American Palaeozoic basin, has 

 helped to confirm the belief of many American geologists in the 

 hypotheses of the metamorphic and metasomatic s:hools. 3 



§ 10. The mineralogical resemblances which exist between the 

 various crystalline terranes above mentioned are easily recognized. 



1 This question is discussed at length by the writer (" Mineral Physiology 

 and Physiography." pp. 457-96) in a study of the geology of the Alps and 

 the Apennines, and of the serpentines of Italy. See als > his paper on 

 " Gastaliii and Italian Geology," containing a hitherto unpublished letter 

 from Gasnldi, in the Geological Magazine for December 1887. 



2 The reader who wishes to follow this question will find it discussed with 

 much detail in the volume already cited "Mineral Physiology and Physio- 

 graphy " (pp. 517-686) under the title of " The Taconic Question in Geology." 

 It is also treated, with some new facts, in the American Naturalist for 

 February, March, and April, 1887, in an article entitled " The Taconic 

 Question Restated." 



