Sept. 27, 1 888 J 



NATURE 



523 



results of metamorphism due to the contact of granite with 

 fossiliferous sediments and the eruptive rocks intercalated 

 therein, on the one hand, and the Archaean schists on the other? 

 For such a comparison useful data are furnished by the 

 Hartz. These mountains, consisting of fossiliferous sediments 

 and the most diversified eruptive rocks, already plicated at 

 the Coal-measure period, represent a fairly average section 

 of the earth's crust, i.e. although there is no axis of crystalline 

 schists, the strata, together with diabases, keratophyres, and 

 the accompanying tuffs, are considerably depressed between 

 highly elevated plutonic rocks (granite, gabbro, &c). 



The contact-zones around the gabbro and granite present the 

 following authigenic minerals : quartz, orthoclase, albite, 

 plagioclase, biotite, muscovite, hornblende, actinolite, augite, 

 bronzite, chlorite, epidote, garnet, vesuvian, tourmaline, axinite, 

 wollastor.ite, cordierite, sphene, spinel, andaludte, rutile, mag- 

 netite, hematite, titaniferous iron ore, magnetic pyrites (pyrro- 

 thine), and other sulphur ores, calcite, fluorite, apatite ; and con- 

 tinued investigations will easily add others to the list, as, for 

 instance, anatase, zoisite, lithionite, lepidolite, corundum, silli- 

 manite, cyanite, graphite — indeed, the four last-mentioned 

 minerals have already been detected in certain mineral aggrega- 

 tions in post-granitic dykes of the Hartz, that probably are to be 

 referred to metamorphic influence. But not only do these minerals 

 show great resemblance to thosi which are most frequently present 

 in ArcJucau crystalline schists ; their combination to definite- 

 mineral aggregates and rocks also makes the analogy even more 

 complete. In the normal gneisses, which are derived, with great 

 diversity of structure, from the culm-greywaekes and the grey- 

 wacke schists 0/ the Oberhartz, in contact with granite and gabbro, 

 are intercalated cordierite- and garnet-gneisses and augite- {or 

 bronzite-) bearing gneisses, which are produced by the alteration of 

 schistose and calcareous sediments. Saccharoidal quartziles are 

 clearly produced by the recrystallization of Carboniferous or 

 Devonian lydites (Kieselschiefer) ; and it is very difficult to dis- 

 tinguish these from rocks produced by the contact-metamorphism 

 of nearly pure quartz-sandstone (Quarzitsandsleine). Horn- 

 stones (corneenue), which contain garnet, amphibole, augite (or 

 bronzite), schorl, andalusite, apatite, as well as orthoclase and 

 plagioclase, are found replacing mica-schists and phyllites. The 

 thin limestone-seams in the Leaver Devonian (Hercynian), Upper 

 Devonian, and the Culm-measure?, are partly metamorphosed 

 to compact or phanero-crystalline " lime-silicate-hornstones," 

 containing garnet or other allied silicates — vesuvian, epidote, 

 malacolite, cordierite, amphibole, sphene, &c. , in places also 

 fluorite or axinite, and corresponding to the garnet-rocks, 

 epidote-rocks, pyroxenites, ecklogites, &c, of the Archaean 

 formation. 



In part, however, they have undergone marmorosis, while 

 being impregnated with garnet or other silicates and locally 

 with ores ; even anthraconite is not altogether absent from these 

 marbles. Amphibolites are in part also derived from calcareous 

 sediments ; those, however, that contain felspar (plagioclase) 

 in any essential quantity can be demonstrated to result from 

 the contact-metamorphism of pre-granilic, Devonian, ami Car- 

 boniferous diabases that have been plicated and metamorphosed in 

 common zvith the strata. Further, there are, in the granite and 

 gabbro contact-zones, alteration- products of the diabase that are 

 rich in biotite ; and other pre-granitic eruptive masses, such as the 

 augite- keratophyres and the augite-orthophyres, show a great 

 abundance of biotite, which is associated with a recrystallization 

 of the orthoclase and of a part of the augite. This biotite is 

 certainly developed at the expense of chlorite derived from 

 auyite or primary hornblende. 



Schistose rocks with more abundant biotite, that are locally 

 present among the more dominant massive rocks, bear the 

 strongest resemblance to garnetiferous mica-schists. In the 

 porphyroids of the Hartz, which occur both within and without 

 the contact-zones, we mainly find sericitic muscovite ; beyond 

 the contact-zone it occurs in such abundance as to produce very 

 schistose sericite rocks, which, on the other hand, are here also 

 derived directly from the porphyritic massive rocks. These 

 porphyroids I regard, from my present stand-point, as the 

 metamorphosed pre-granitic tuffs of quartz-keratophyres and 

 quartz-porphyries. To these tuffs are perhaps related certain 

 hornstones, very rich in orthoclase, which occur in the granite 

 contact-zone with Devonian and Carboniferous siliceous schists 

 (equivalents of Adinole ?). 

 Other questions are : — 

 (2) What differences exist in the order of crystallization of the 



minerals which compose granites, quartz-diorites, gabbrds, 

 diabase c , in short holo- and phanero-crystalline eru| tive rocks, 

 and that of the secondary minerals produced in the contact' 

 1 metamorphism of these eruptive rocks? 



This question must \s the more carefully answered, as, in 

 I spite of the rich material so excellently collected and cleverly 

 • arranged for the use of science by H. Rosenbusch, the order 

 I of crystallization of the eruptive rocks is not yet firmly estab- 

 lished. A certain degree of regularity is undeniable ; but, on 

 the one hand, the chemical law is, as Ligorio has demonstrated, 

 more intricate than that formulated by Rosenbusch ; and on the 

 other, the order varies quite unaccountably with alterations in 

 the physical conditions of consolidation (compare granite and 

 pegmatite). 



(3) Is the ophitic (diabase-) structure under all circumstances the 

 structure of an eruptive rock, or are there undoubted sedimentary 

 rocks possessing a similar structure? 



(4) It has been proved that graphic granite, as micro- and macro- 

 pegmatite, forms an integral part of true eruptive rocks, espe- 

 cially of granite and its porphyritic modification. Since graphic 

 granite is very common among the gneisses, the question arises 

 whether it is to be regarded as a true eruptive rock, or whether 

 such occurrences can be proved to have been produced by 

 thermal action, or even lateral secretion, in the sense of a partial 

 solution of the neighbouring rocks. 



Even if it be admitted that all minerals can be produced, by a 

 suitable variation of the conditions, either by consolidati >n, by 

 separation from aqueous solutions, or by 'sublimation, still it 

 does not follow, to my mind, that all the structures that com- 

 bine minerals to regular aggregates, can be produced in like 

 manner in these three modes of formation. It seems to me that 

 such structures — as, for instance, the ophitic (diabasic) or the 

 pegmatitic (to say nothing of the structures which are deve- 

 loped in rocks containing glass or other base) — that have been 

 demonstrated to be characteristic of rocks of undoubted eruptive 

 origin, must rather be regarded as indicating an origin by con- 

 solidation from a magmatic condition, so long as contrary proofs 

 are not forthcoming. No one, to my knowledge, has ever main- 

 tained that the ophitic or diabasic structure can be of sedimentary 

 origin ; but gabbros have been claimed — wrongly, as I believe — 

 as sediments, in spite of the close relation of their structure to 

 that of the diabases. 



As regards graphic granite (or macro-pegmatite), the case is 

 somewhat different. 



The frequent occurrence of such masses in gneiss has created 

 the notion that they are integral components of the sediment- 

 ary gneisses. And this view is maintained, although a con- 

 siderable portion of these pegmatitic masses can be clearly 

 seen filling vein-like cavities, while another part make up lenti- 

 cular patches that follow, more or less, the dip and strike of the 

 schists. The occurrence of simple aggregates of quartz and 

 feldspar, that are of thermal origin, must* then, in accordance 

 with one's experience of regional and contact-metamorphism, be 

 unconditionally conceded ; while the absence of such aggregates 

 in the greywackes appears to me to absolutely disprove a deve- 

 lopment by lateral secretions. It is therefore not inconceivable 

 that the pegmatitic aggregates represent, so to speak, the quint- 

 essence of the gneiss, exuded into primary cracks. At the same 

 time, great caution is to be recommended ; for, since the intro- 

 duction of the microscope, micropegmatite has, little by little, 

 been recognized as an essential constituent of numerous acid and 

 basic (with Si0 2 per cent, as low as 48) rocks. The veins of 

 graphic granite in the Hartzburg gabbro have been held by 

 some for segregation-veins. They are, however, demonstrably 

 apophyses of the eruptive granite ; indeed, the principal mass of 

 granite in the Brocken massif is, in the main, micropegmatitic. 

 The banded stnicture, with bilateral symmetry, of many peg- 

 matites, which has been compared to that of many mineral 

 veins, is no proof of their non-eruptive nature. The augites, 

 felspars and other minerals of lavas present banded structures 

 with variable chemical composition : banded structure with a 

 chemical composition varying from that of diabase to granite- 

 porphyry, is shown by compound eruptive dykes, as has 

 lately been well shown by Bucking, in the Thiiringerwald 

 ("Jahrb. d. kgl. preuss. Geol. Landesanst. f. 1887," p. no, 

 et set/. ). Even the drusy character and the richness in minei'als 

 presented by the central portion of many pegmatite-dykes finds its 

 analogy in the external shells of true eruptive granites, which 

 may, however, be complicated by the influence of thermal actions, 

 accompanying, or subsequent to, eruption. Giant spherulites, 



