arrangement, and of tourmaline, they approach in character the socalled older phyllites,' from which they are distinguished by their finer grain. The induration and the fine puckering of some calcareous rocks associated with this group are no doubt due to contactalteration by the neighbouring igneous rocks. Dark red quartzitic rocks are found in several places, along with the green schists. The green quartzitic rocks often exhibit sharp little folds marked by thin mineral bands, which may indicate the original beddingplanes. A strong southerly dip, due to foliation, predominates over the whole of the Upper Tarnthal between the Sonnenspitz and Röckner ridges. The Nederer section terminates with the quartzitic schists. No actual occurrence of serpentine has as yet been observed on this ridge, but the relation of the serpentine to the schists is well shown in the neighbouring Upper Tarnthal and on the Röckner ridge. Between the altered schists and the serpentine are seen some remarkable forms of ophicalcite.' Masses of calcareous schist, several feet in thickness, have been injected along the foliationplanes with the basic magma now represented by serpentine. Thin bands and lenticles of serpentine alternating with the schist impart to the rock a parallel structure, which gives the appearance of true bedding and suggests a relation of conformity with the schists below. These are, no doubt, some of the occurrences that led Pichler to include ophicalcite and serpentine in a conformable succession of sedimentary rocks. But at Matrei and Pfons, where the rock-series resembles that of the Tarnthal, the ophicalcite is not in a form such as to suggest the explanation here given of its origin. The parallel structure is absent or imperfectly developed, and the calcite often appears in veins. The mode of formation of these mixtures of serpentine and calcareous rock is evidently subject to variation. The general dip of the schists under the serpentine-mass of the Röckner led Prof. Rothpletz to the conclusion that the serpentine was in the form of a sill resting upon the syncline of schists. But the intrusive masses are very irregular in shape. This is well seen in the case of the small serpentine-mass in the Upper Tarnthal, on the southern slope of the Sonnenspitz ridge. In view of the more recent hypotheses of Alpine structure, which assume a translation from a distance of some or all of the rocks composing this mass, it seems important to distinguish between characters impressed on the rocks in their original site (here called œcogenous characters) and those which may have been acquired during the movement (apœcous characters). Superimposed or Apœcous Characters of Rocks The distortion and partial foliation of the bedded limestones just above the dolomite may have been imposed during a movement of translation, but it does not appear that the fossils here are more deformed than those from the Lias of Southern Tyrol, where there is no question about the rocks being in their original site. But the conspicuous mechanical shearing of the calcareous rocks just above the bedded limestones, as also the strong folding and crushing in the region of the dolomite-breccia, may and probably has been acquired during movement. Original or Ecogenous Characters of Rocks The topmost layer of quartzite and other altered schists and the serpentine bear many characters which must have been acquired before any movement of translation began. The parallel arrangement of the minute plates of chlorite in the quartzitic rocks (which also contain tourmaline) is a feature that must have been acquired at deep levels, under conditions similar to those determining the structure of the older crystalline phyllites. The tourmaline is in the form of single, undistorted, minute rods, bounded by faces of the prism and often showing a terminal pyramid-characters which prove it to be the most recent addition to the minerals composing the quartzitic rock. The close vicinity of rocks, presently to be described, which have been modified by contact with the serpentine, suggests that the tourmaline may be a contact-mineral. The serpentine, however, does not contain tourmaline. If not a contact-mineral, the tourmaline must be older than the serpentine-intrusion. In either case the tourmaline, the youngest product, was formed before the rock-mass left its original site. Serpentine. The association of green schists with serpentine in several distinct occurrences (Mieselkopf, Matrei) has been noted by previous observers, as well as the occurrence of talc and magnesian minerals in the schists adjoining the serpentine.' The igneous intrusion appears to have commenced at a time when the rocks were still under the influence of causes producing foliation. Recrystallization under stress is indicated by the formation of talc in the schists, by the presence of sheafy amphibole in parts of the serpentine-mass near the contacts. That violent movements were in progress during the period of activity covered by the serpentine-intrusion is shown by the mode of injection of the serpentine in the ophicalcite, and by the numerous detached masses of altered calcareous rock found embedded in the serpentine at all levels up to the summits of the Röckner and the Little Röckner. Among these torn-off fragments 1 See J. Blaas, Ueber Serpentin & Schiefer aus dem Brennergebiete' Nova Acta Leop.-Carol. Akad. der Naturforscher, vol. Ixiv (1894) no. 1. are some which show the ophicalcitic structure, and thus bear witness to the ingress and complete consolidation of a portion of the serpentine-magma at a date prior to a final stage of the eruption in which the fragments were detached and carried off. Last Stage of the Intrusion. The final accession of fluid magmas is represented by a core of serpentine which has crystallized by slow cooling in a state of rest, unsolicited by forces of shearing or stress. This is proved by the abundant remains of large pyroxene-crystals showing no direction in their arrangement, and by numerous pseudomorphs in bastite after pyroxene in composite crystalline growths which evidently still retain their original forms. In the younger parts of the rock an occasional slight strain in the pyroxenes and some instances of cataclastic structure are the only indication of the survival of forces, which were at the last too attenuated to impose schistose structure on the rock. The rock must have consolidated nearly in its present form while still in communication with the main magmatic reservoir, and before the commencement of the journey from the original to the present site. The original igneous rock has been affected chemically, notably by serpentinization on an extensive scale. Some of the changes may possibly have taken effect during translation, but the change of form has been unimportant. The ophicalcite obviously acquired its banded structure on the original site, and has been translated without perceptible deformation. The same is true of the masses of indurated contact-schist. It appears to me to be evident that the whole mass of serpentine, ophicalcite, and indurated schist still hangs together with the original relative positions of its parts, and that in the course of translation it has undergone no deformation and no interruption of continuity, beyond that due to minor faults and fractures. SUMMARY OF CONCLUSIONS. As respects the general structure of the Tarnthal mass, my reading of it is as follows: The rock-series of the mass may be divided into three parts : (1) A lower section consisting of (1 a) principal dolomite (Rhætic) and (16) Liassic limestone, the upper beds being the youngest. This lowest portion is in normal position, and is scarcely disturbed. (2) A middle section consisting of (2 a) calcareous schists, (2b) a band of massive dolomite and dolomitic breccia, and (2 c) calcareous schists with green bands. This section shows marks of violent distortion and crushing. (3) An upper section, consisting of more or less altered quartziteschists, with calcareous schists, ophicalcite, and serpentine. This section retains most of its original character and form, and has undergone little mechanical disturbance since it left its root.' This is summarized in the following synopsis : Serpentine. ZONE 3 Ophicalcite. Tarnthal Quartzites, etc. Calcareous schists with green bands. ZONE 2 Dolomitic breccia. ZONE 1 Calcareous schists. f Liassic limestone. Principal dolomite (Rhætic). 6 The explanation of the structure now suggested is as follows:The distinct line of division between the bedded limestones and the calcareous schists-that is, between Zones 1 & 2-marks approximately the lower limb of a long fold, the dolomitic breccia being thus a repetition in a highly attenuated form of the principal dolomite below. As regards the relations between Zones 2 & 3, the interpretation is not so clear; the absence of any line of demarcation between these two series of rocks, which have been affected in different degrees by dynamic activity, gives rise to some difficulty. But the hypotheses that present themselves are: (a) The collective mass 2 & 3 is in inverted sequence, the serpentine and green schists belonging to an older series normally and immediately below the dolomite-the dolomitic breccia belonging to the lower limb of a fold, the upper limb of which is marked by an air-line above the serpentine. Or (b) The dolomitic breccia represents the whole of the principal dolomite in a flattened fold, the nappe. The serpentine and quartzites have been brought into their present position by a long overthrust, the traîneau écraseur of Prof. Termier. The relation of 1 and 2 in this case is one of enforced conformity, instances of which are known elsewhere. 2 s Q. J. G. S. No. 256. 31. NOTES on the GEOLOGY OF BURMA. BY LEONARD V. DALTON, B.Sc. (Communicated by Dr. A. SMITH WOODWARD, F.R.S., F.L.S., V.P.G.S. Read April 15th, 1908.) [PLATES LIV-LVII-FOSSILS.] Ir is the object of this paper to present the results of geological expeditions in the Irawadi valley carried out by my uncle Mr. W. H. Dalton, F.G.S., and myself in the season of 1904–1905, and by myself alone in 1905-1906, and to correlate these observations with those made by previous writers, thus summarizing our present knowledge of the geology of Burma generally and of the Tertiary System of that country in particular. To this end it is divided into the following sections: Dr. Fritz Notling in 18951 reviewed the work done on the geology of Burma before that date, pointing out the remarkably-accurate hypothetical arrangement of the rocks of Burma made by Buckland in 1828,2 and the absence of subdivision of the Tertiary System previous to W. Theobald's paper on Lower Burma in 1873. In this summary the Axial Group' of Theobald is referred to as certainly not of Triassic age a statement controverted by later examination of the fossils of the Arakan Yoma and other parts of Lower Burma— although found true in the sense that the group is a complex one (as we now know), and neither Triassic nor Cretaceous alone, but assignable to both of these. While Dr. Notling's paper thus reviews the greater number of the works published on the geology of Burma, and in particular those relating to the Tertiary of the better-known regions of the Irawadi basin, there are certain other papers to which it is necessary to refer here, as they deal with the outlying districts. Of these the earliest is that of E. J. Jones on the coals of the Chindwin Valley, wherein he describes the 1 Rec. Geol. Surv. India, vol. xxviii, p. 59. 2 Trans. Geol. Soc. ser. 2, vol. ii, pt. iii, p. 377. The fossils collected by J. Crawfurd near Singu and Yenangyat on the Irawadi, referred to by Buckland, are in the Museum of the Geological Society, and apparently include several of the forms named many years later by Dr. Nætling. They are of especial interest, as being the first fossils from Burma to reach this country. On the Geology of Pegu' Mem. Geol. Surv. India, vol. x, p. 189. 4 Notes on Upper Burma' Rec. Geol. Surv. India, vol. xx (1887) p. 170. |