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in conglomerates of Miocene age. The serpentines are of a normal type, with olivine, when any still remains, altering into chrysolite, magnetite and chromite in crystals, grains, irregular concretions and veinlets.1
On the geological sketch map of the country north of Bhamo by C. L. Griesbach and F. Noetling, which illusOther intrusive ser- trates the latter's account of the jadeite occurrences, twelve distinct and large intrusions are shown between Bhamo and Myitkyina, as Jadeite and Basalt?", as well as two smaller ones in the vicinity of Tawmaw.2 M.
Stuart has indicated the presence of smaller ones still between Myitkyina and Putao.3 Although these ultrabasic intrusives of the Irrawaddy valley proper lie outside the geographical limits of the main Arakan Naga Province, they are probably identical in age and origin with its intrusive serpentines.
THE PEGU GULF.
The Pegu Gulf is the name given by E. H. Pascoe to the Central tract of Burma lying between the foot-range of the Arakan Yoma on the west and the highlands of the Shan plateau and their southern prolongations on the east.4 It may eventually prove to cover the little-known Tertiary deposits of the Hukong Valley which may be connected with those of Upper Assam through the Dihing basin and may, again, be disconnected from those of the Upper Chindwin.
The Pegu Gulf contains the whole sequence of the Burmese Tertiaries ranging from the Lower Eocene
Orography. through the Miocene and Oligocene to the Pliocene, a thickness of marine, estuarine, fluviatile and aeolian deposits totalling some 41,000 feet. The gulf stretches some 600 miles from north to south and is, on an average, about 100 miles wide. It contains a great part of the present valleys of the Irrawaddy and Chindwin and also the valley of the Sittang, separated from that of the Irrawaddy by the low, north and south trending ridge of the Pegu Yoma which reaches from the Meiktila district to Pegu,
1 A. W. G. Bleeck: (5), pp. 254-285.
2 F. Noetling: (50), pp. 26-31.
3 M. Stuart: (63), p. 247.
4 E. H. Pascoe: (56), p. 23.
5 G. de P. Cotter: (21), p. 415.
a distance of approximately 250 miles. The higher ground of the Pegu Yoma generally rises over 1,000 feet and but seldom crosses the 2,000 foot contour. From the sea coast, narrowing gradually towards the north, in typical deltaic fashion, the alluvium of the Irrawaddy rises very regularly but almost insensibly; nearer the sea the land is low and rarely above the high level of spring tides. The spurs from the Yomas and the ridges such as the Pondaung Range parallel to it, break the monotony of the lower ground further north. Owing to the manner in which the Arakan Yoma follows the Burmese Coast and prevents the moisture-laden winds of the south-west monsoon from reaching the districts to the east, there is an extraordinary variation in the climate of the Tertiary belt. In the south of the delta there is a mean annual rainfall of over 100 inches, diminishing to 25 inches per annum about latitude 21°, 250 miles away, and gradually increasing again further north. These conditions have a striking effect on the ecology of the riparian tracts of the Irrawaddy and tend still further to diversify the scenery.1
It is impossible here to enter into details of the geology of the Pegu Gulf or to mention even briefly the work Geology. of all those who have participated in its elucidation. The foundations were laid by W. Theobald and have remained firm, with very little modification, up to the present time. The superstructure was raised by E. H. Pascoe in his monumental work on the oilfields of Burma2 while the continuation of the geological mapping has been carried out by various officers of the Geological Survey of India. The systematic study of the fossil remains. found in the Tertiary rocks was commenced by F. Noetling and carried on in a most detailed and critical manner by E. W. Vredenburg until his untimely death in 1923. G. de P. Cotter has described the Lamellibranchiata of a portion of the Eocene and has interpreted the geotectonics of the Gulf. L. Dudley Stamp has recently summarised the results of many previously published and scattered observations which tend to prove that :
(a) the Tertiary succession is predominately marine in the north and mainly continental in the south.
1 L. Dudley Stamp and Leslie Lord: "A Preliminary Note on the Ecology of Part of the Riverine Tract of Burma". (To appear shortly in the Journal, Asiatic Society of Bengal.)
2 E. H. Pascoe : (56.)
(b) each horizon can be traced laterally from marine through deltaic into fluviatile or aeolian deposits as one proceeds northwards.
(c) there is on the whole, a tendency for continental conditions to move southwards as one ascends in the succession.1
It is now proved that the Irrawaddy basin was a subsiding geosyncline during the Tertiary period and while the Pegu Gulf was being slowly filled in by the sediments of one or more big rivers from the north, resulting in a gradual and general retreat of the sea to the south, there were periodic marine invasions from the south which have left their remains in wedge-shaped masses of marine deposits amongst rocks of genuine continental types. The effects of minor folding and buckling of the floor of the Gulf are sometimes found but are nowhere more apparent than around Shinmadaung near Pakokku, where a ridge of pre-Cambrian gneisses is overlain by Miocene rocks. The movements of uplift along the line of the Arakan Yomas and the accompanying subsidences of the floor of the Pegu Gulf itself culminated in the pronounced and widespread folding of Pliocene times.
The occurrences of petroleum on the Arakan Coast need not be considered here. They are of no great importance as far as is known at present and form part of the southern continuation of the oilbearing belt of Assam. Yenangyaung the most productive oil-field of Burma, lies some two miles to the east of the Irrawaddy at Yenangyaung in the Magwe district. The oil-bearing strata form a symmetrical, elongated dome, about 6 miles long and 1 mile broad, but the producing area does not cover more than 1 square miles. Although the shallow oil sands of the Pegu rocks of this field have been worked by the Burmese from a remote antiquity, its modern exploitation dates only from 1887 when the Burmah Oil Co., Ltd. commenced drilling. Some of the wells are not over 3,000 feet in depth and the field keeps up a fairly steady output in the neighbourhood of 200 millions of gallons per annum.
Oil Fields of the Pegu
The Singu oilfield, in the Myingyan district, lies further north but on the same bank of the Irrawaddy as Yenangyaung; it is really the southern prolongation of the anticlinal fold that has given rise
1 L. Dudley Stamp: (50), pp. 441-501.
to the Yenangyat field. It was discovered by an officer of the Geological Survey of India in 1898 and it contains rich oil sands. between the levels of 1,400 and 1,450, and 1,800 and 1,900 feet below the surface, while its output reaches over 100 millions of gallons per annum.
The Yenangyat oil-field has a length of 39 and a maximum width of 3 miles, running for the greater part of its length as a steep scarp along the eastern bank of the Irrawaddy, in the Pakokku district. It forms a pronounced, asymetric anticlinal fold, the eastern limb of which is highly inclined, vertical or even inverted. Yenangyat began to produce in 1893 and obtained a maximum yield of over 22 millions of gallons in 1903, but there has been a marked. decline since then and the field now gives about 2 millions of gallons per annum.
The upper Chindwin oil field has been prospected for some years and entered the list of productive fields in 1918 with an output of almost half a million gallons. This has increased to over 1 million gallons in 1921.
An acute anticlinal fold commences in the Pegu rocks, 2 miles to the north of Minbu and extends for several miles to the southsouth-east. For a distance of 14 miles south of the town it is buried under alluvium but further south still, it appears as a broad arch which develops a subsidiary fold as one proceeds along its strike. Near Minbu itself there are mud volcanoes and gas pools but deep. drilling has been somewhat disappointing for a number of reasons. Production commenced in 1910 with an output of 18,000 gallons, it attained a maximum of 4 millions of gallons in 1918 which had fallen to about 3.75 millions of gallons in 1921.
The small oil field of Padaukbin lies on a flat-topped, asymetrical anticline, 3 miles north-west of Thayetmyo; it has yielded small quantities of oil for many years and is now being deep drilled. The small field of Yenanma is also in the Thayetmyo district. In addition to these producing oilfields petroleum is known to occur in seepages, gas pools and springs at many other places amongst the Tertiary rocks of the Pegu Gulf.
Stuart has described coal seams in sandstones of Lower Tertiary in Coal in the Pegu Gulf. age at Hlemauk, Kywezin and Posugyi the Henzada district. Though seams up to 10 feet in thickness are known, the beds containing them are very
contorted and faulted.1 Thin seams of poor quality are also known to occur in the Kyaukpyu and Thayetmyo districts, again in Tertiary rocks but they, are of little value. In the Pakokku district there are two small coal-bearing areas. In that of Letpanhla numerous seams are exposed and have been examined over 1 miles. The main seam is from 5 to 6 feet thick but contains many bands of shales. There are three thin and variable seams of fair quality coal in the Tazu area but here again partings of shale are frequent. The economical value of these coals is very problematical. The seams in both cases occur in the lowest sub-division of the Pegu series and are of Miocene age.4
Coal-bearing rocks of Tertiary age have a wide extension in the valleys of the Nantahin, Peluswa, Maku and Telong streams, to the north of Kale, in the Chindwin valley, for a distance of 55 to 60 miles. The Nantahin-Peluswa tract covers an area of about 25 square miles, with a total thickness of coal of 48 feet. The field has not yet been developed though a few hundreds of tons of coal have been removed from a 10 foot seam exposed near Kale.5 The thin and unimportant coal seams of Katha and Bhamo districts are also probably of Tertiary age.
Small quantities of amber have been found in the Yenangyat oil field from time to time, but the 6 wellknown, fluorescent, brown amber of Burma, which has been known to the Chinese for centuries comes from Maingkwan, an isolated village in the Hukong valley where the mines were first visited by the Europeans in 1836. The fossil gum occurs in irregular lumps in a blue clay, and it often contains the remains of insects entrapped in it as it exuded from its parent trees. A recent study of the insects has led to the belief that the amber is of Eocene age. The connection of the Tertiary strata of the Hukong valley with the Pegu Gulf proper is obscure and will not be definitely known. until the detailed geological survey of the upper Chindwin valley has been made.