THE MOGOK-FRONTIER REGION. In this group I include the gneisses, crystalline limestones and intrusive granites of Upper Burma and the northern part of the Burma-China frontier, separating from them in another group the highly metamorphosed sediments comprising the mica schists, phyllites, slates, quartzites, etc., which are known as the Monglong and Chaung Magyi Series in the Shan States and the Kao-liang Series in Yunnan. The rocks are typically developed in the Ruby Mines district of Upper Burma where their southern bounOrography. dary is fairly well defined by the valley of the Nam-pai. To the north of this river they extend in a succession of parallel ranges from north-east to south-west which is also the strike trend of the rocks themselves. Proceeding northwards the crests rise from elevations of 2,000 to 3,500 feet to heights of 4,000 to 7,000 feet above the sea, until they reach 7,544 feet in Taung-me, immediately above Mogok.1 The north and north-eastwards extensions of these rocks are still to be mapped in detail but they are known to be continuous with the gneisses of the country north of Bhamo, seen by Griesbach and Stuart and with the biotite gneisses. and crystalline limestones of the frontier ranges between Bhamo and Tengyu-h.2 area In north-eastern Myitkyina Stuart found intrusive granite-gneiss with strips of crystalline limestone running north and south and penetrated by numerous veins of granite.3 Griesbach noted a coarse, porphyritic gneiss characteristic of the eastern portion of the he reported on accompanied by schistose, phyllitic and hornblendic varieties, the whole being extensively folded and crumpled and bearing seemingly conformable crystalline limestones in the flexures. Both Griesbach and Stuart seem to consider the crystalline limestones as metamorphosed Palæozoic rocks, a view which is not acceptable to the writer in the absence of further evidence. Writing of the Archæan sequence between the Ruby Mines and Yunnan, M. H. Loveman states that the gneiss is present in a great variety of types and often assumes both schistose and granitoid forms. All phases from extremely acidic to basic hornblende gneisses are found. This area of igneous and metamorphic rocks maintains a great 1 T. H. D. La Touche: (39), p. 33. 2 C. L. Griesbach (28), pp. 127-130; J. Coggin Brown (9), pp. 173-205. 3 M. Stuart: (64), p. 406. regularity in its diversity, whether examined far to the south of Mandalay, (east of Pyawbwe) in latitude 24° 401, or in the north (at Teng-yüeh) in latitude 25°. The practical continuity of the bands of coarsely crystalline limestone in a well-defined zone, more than 250 miles in length is held by Loveman to be an almost conclusive proof of its original, organic origin.1 In the Ruby Mines district the width of the gneissic belt is at least 40 miles, about the latitude of Mogok, and to the south it fills in the narrow strip between the Nampai and the Irrawaddy. In this area the strike is still north-west, but on the western bank of the Irrawaddy it turns due north and south and the Archæan rocks build the narrow range of hills which runs parallel to the river and terminates near Sagaing, opposite Mandalay. On the eastern bank of the Irrawaddy, the Paleozoic rocks come right down to the plains and the Archæan rocks are only to be seen in a few outlying hills rising abruptly from the alluvial carpet, but they re-appear again at Kyaukse, 25 miles south of Mandalay and beyond this point may form a more or less continuous band, from 12 miles and upwards in width, along the edge of the Southern Shan plateau Of the easily accessible regions of Burma, less is known of the rocks of this part than any other and we are hampered in an attempt to decide which of our regions it belongs to. How much of the band, which stretches almost to the sea in the vicinity of the mouth of the Sittang really belongs to the Archæan group, or what portion comprises later granitic intrusions, will not be known until the detailed survey of Burma has progressed considerably further than it has done to-day. East of Kyaukse P. N. Dutta found a group of old, metamorphosed sedimentary rocks, perhaps of Paleozoic age, intimately associated with and altered by intrusive granite.2 C. S. Middlemiss found well-foliated micaceous and hornblendic gneisses, penetrated by syenite granites and pegmatites on his traverse from Thazi to Taunggyi,3 and writes of the towering, sharp-edged ridges of the western boundary of the gneissic and metamorphic zone rising in a straight, unwavering line from the alluvial plains of the Meiktila, Yamethin and Toungoo districts. A. M. Heron in a personal communication to the writer states that while crossing from Tatkon, a few miles south of Yamethin to 1 M. H. Loveman: (42), p. 108. 2 H. H. Hayden: (30), p. 29. 3 C. S. Middlemiss: (47), pp. 128-129, Byingyi, a high granite peak overlooking the plains, he observed no gneisses or similar rocks. W. Theobald mapped the western margin of the crystallines in Toungoo and Pegu where they comprise his Martaban Group, and although he was inclined to regard most of them as gneisses he was puzzled by the manner in which "the schistose and crystalline characters of the beds are seen to give place insensibly to the granitoid habit." 1 It is noteworthy that no crystalline limestones of the Mogok variety have been reported from these southern regions. For our present purposes and in the absence of further knowledge we tentatively regard the Mogok-Frontier Province as ending about the latitude of Pyawbwe, or a short distance to the south of it, and the crystalline rocks of the regions further south as probably mainly intrusive granitic types belonging to our Tenasserim Province. Characters of the The gneisses and associated rocks of the Ruby Mines were studied by C. Barrington Brown and J. W. Judd.2 The general mass consists of biotite gneisses, biotite-granulites gem-bearing limestones and, more rarely, biotite schists. Hornblende and associated rocks. is usually absent and garnet exceedingly common. Interfoliated with these intermediate types are other rocks of a more acidic nature, including very coarse pegmatites and graphic granites, aplites and granulites, granular quartzites and orthoclase-epidote rocks. The orthoclase frequently contains inclusions of fibrolite and is often converted into moonstone. In the Nyaungok district rubellite and indicolite often of fine gem quality occur in them. There are other ubordinate basic and ultrabasic rocks, pyroxene gneisses and granulites with basic felspars related to anorthite in which a partial or complete transformation into scapolite is sometimes traceable. The ferro-magnesian silicates are represented by sahlite, diopside and aegirine, by bronzite, hypersthene and rarely by hornblende while garnets are frequent and abundant. The leading types are augite gneiss, enstatite gneiss, enstatite granulite, scapolite gneiss, pyroxenites, amphibolites and lapis-lazuli (lazurite-diopside-epidote rock). Many of these contain crystals of calcite scattered through them. The crystalline limestones which contain the rubies, sapphires and spinels are most intimately associated with the basic rocks and the passage from one into the other is of the most insensible kind. Some of the rubyW. Theobald: (65), pp. 140-141. : 2 C. Barrington Brown and J. W. Judd (7), p. 387. G bearing limestones are highly micaceous while others are calciphyres. With the gems, many oxides and silicates, both original and secondary, occur, and, in addition much graphite and pyrrhotite. Prof. Judd believed that the crystalline limestones were formed by purely chemical processes operating at great depths in the earth's crust, a view not accepted by LaTouche and other geologists, who think that they were originally components of a sedimentary series associated along a particular zone with rocks of igneous origin. Minute flakes of graphite are freely disseminated through the crystalline, ruby-bearing limestones, and in some places the mineral is concentrated into lenticular beds, developed along the line of contact of the limestones with scapolite gneiss. Attempts to mine this mineral, however, have not been successful.1 The best known localities are at Wabyudaung and Kyaukgyi. Graphite in the crys talline limestone. The ruby-bearing limestones crop out from the alluvium of the Irrawaddy valley in the Sagyin Hills, a few Ruby-bearing limestones of the Sagyin miles north of Mandalay, and some years ago Hills and Myitkyina. rubies, sapphires and spinels were regularly obtained there; later, however, the workings were closed. There are also areas in the Naniazeik neighbourhood of the Myitkyina district where the same gems have been found. Naniazeik itself is about 90 miles east-south-east of Tawmaw, the jade-mining centre. The precious stones used to be obtained from the detritus afforded by the disintegration of crystalline limestones surrounded by intrusive masses of granite.2 According to J. Gold in the Mogok Frontier Province. M. Maclaren large, heavily mineralised but very low grade gold-quartz veins are reported to occur in the gneissic range lying south of the Shweli River in the Northern Shan States.3 THE CHAUNG MAGYI GROUP. Herein are included both the mica schists of Mong-Long and the rocks of the Chaung Magyi Series. These rocks occupy the broad area of broken, hilly country which intervenes between the gneisses of the Distribution and Geo logy. 1 T. H. D. LaTouche: (40), p. 220. Ruby Mines District and the Shan plateau. The mica schists spread across a somewhat restricted area in the State of Mong-Long and are nearly always of a biotitic nature. They are traversed everywhere by barren veins of milk-white quartz and along the boundary with the gneisses frequently have thick dykes of intrusive tourmaline granite. There is no sharp boundary between these mica schists and the phyllites, slates, greywackes and quartzites of the Chaung Magyi Series. The latter were consolidated, folded, dislocated and denuded before the earliest Palæozoic sediments were deposited upon them, and they are exposed in those places where the latter have been removed, as, for example, along the western edge of the plateau and amongst the hills to the north of the Gokteik gorge. They also appear along the eastern margin of the plateau in the ranges between it and the Salween as shown on the map, in the huge mass of Loi Ling and in the hills to the east of Mongyai, where they occupy the cores of elongated domes. Another detached area is found to the south of the plateau forming the mountain masses of Loi Pan (6,693 feet) and Loi Twang (5,752 feet), which like the other inliers rise as islands from the plateau surrounding them. Great intrusions of granite, in which tourmaline is rare or absent, are found amongst these rocks in places. They occupy a large area around Nam Hsan, the capital of Tawng Peng, and occur on the northern side of Loi Pan and along the southern slopes of Loi Ling. Mong-Long. The gem tourmalines which used to come from Mong-Long were obtained from thick beds of gravelly Gem Tourmaline in detritus on the hill-slopes to the north of the the valley of Nampai, 2 or 3 miles north of town of Mong-Long and are believed to have been derived from the weathered products of several broad veins of the granite already described.1 Perhaps the tourmaline mines of Maingnin, described by E. C. S. George, which are situated in the State of Mongmit to the north of the Ruby Mines are in a similar formation.2 Several quartz veins carrying iron pyrites in large quantities Pyrites in the Chaung occur at Hungwe in the Tawng Peng State,3 Magyi Series. in slates of this series. 1 F. Noetling (49), pp. 125-128. 2 E. C. S. George: (26), pp. 233-238. 3 H. H. Hayden (31), p. 24. |