surface area than any other group of rocks, it has attracted the attention of every geologist who has visited the province, yet its age and relationship to similar strata in adjoining regions still remain matters of controversy.

Red formations generally attributed to one or more of the Mesozoic systems, or to some division of the Tertiary period, occur in many parts of South-Eastern Asia, but limiting our range to the territories close to Yunnan, they are found along the eastern shores of the Bay of Bengal, on the Mergui Coast; in the north-eastern parts of the Amherst district of Burma, through Northern Siam into the Federated Shan States and thence across Yunnan into Kueichow, Kwanghsi and Kwangtung. Again, from the Red Basin of Szechuan on the north, through Yunnan into the French provinces of Indo-China, they cover immense areas and stretch for unknown distances into Eastern Siam. (Pl. 29.) The unfossiliferous character of most of these sediments has led to widely divergent views of their ages and rendered their correlation an unusually difficult task. Yunnan may be regarded as the centre of the great region thus roughly indicated and any evidence which it can supply should help towards a solution of the problems which the Red Beds present further afield.

Basin of Yunlung.

Crossing Yunnan from the direction of Burma by the BhamoTali Fu-Yunnan Fu track, the Mekong gorge itself is excavated in the sub-metamorphic rocks of the PrecamThe Subsidiary Red brian Kaoliang Series. These are at once overlain by red sandstones and shales outcropping with monotonous regularity for some 40 miles further to the northeast, to be followed either by the crystalline rocks of the steep western flanks of the T'sang Shan range near Tali Fu (25° 40': 100° 10′), or by the old Palæozoic limestones of their lower slopes. (Pl. 30.) Northwards from the track similar strata extend in an unbroken sequence for at least 70 miles. I shall refer to this particular spread of Red Beds as the Yunlung Basin, from the town of the same name (25° 47′ 99° 19′), the centre of one of the saltproducing regions of Yunnan, for this mineral is obtained in large quantities either as brine or as rock salt, from certain horizons of the formation. It is doubtful if the Yunlung Basin is a separate entity as some geologists have supposed; it is more probably a subsidiary prolongation towards the west of the main basin of Central Yunnan.

Other Occurrences.

This central basin is best known from the exposures along the Tali Fu-Yunnan Fu (25° 3' 102° 40') track on which they are practically continuous from the eastern edge The Central Basin and of the Triassic Gulf of Yunnan-i, at Annankuan (25° 23′ : 100° 51′) to Anning Chou (24° 56′ : 102° 29′), a distance of approximately 110 miles in a straight line. As it is along this route that most of the observations of recent years have been made and regarding which differences of opinion have arisen, the present account will be largely devoted to it. It would be misleading, however, to allow such centralization to obscure the fact that the Yunnanese Red Beds cover a far vaster surface than even this long section indicates. In the north of the province, across the Yangtze, I followed them from the river to the neighbourhood of Yungpei T'ing (26° 42′ : 100° 45′), a direct distance of over 30 miles, without finding their northern limit. In the far south, around Szemao (22° 48′ 101° 3'), they are known to stretch to the Mekong at least, though again broken by a narrow zone of marine Triassic deposits; further south still their limits are again undemarcated though they probably cross from China into the Laos; moreover, I think that they occupy much of the course of the Papien Ho, or Black River, in Yunnan, from about the latitude of P'uerh Fu (23° 4′: 101° 4′), to its head about latitude 24° 50'. Thence they cross its watershed into the upper valley of the Yuan Chiang, or Red River, a distance of over 180 miles as the crow flies from north to south; continuing further north they appear to wrap around the southern termination of the T'sang Shan* and so to join the subsidiary basin of Yunlung.

West of the karstic regions of Eastern Yunnan, in every direction traversed, both to the north and to the south of the Tali FuYunnan Fu route, I met the Red Beds and there is little doubt that when the intervening areas come to be surveyed, they will be found to cover much of Central Yunnan, geologically unexplored up to the present time.

The Red Beds can be divided into three major divisions. The lowest consists of thick conglomerates containing pebbles of rolled quartz and limestone, followed by alternations of sandstones and shales of various bright tints. Thin laminae of a marly nature are

The Threefold Division of the Red Beds.

* The T'sang Shan is the name of the high mountain ridge of Archæan rocks, rising to 14,000 feet, immediately to the west of Lake Erh Hai, on the shore of which Tali Fu is situated.

sometimes present. Good examples of this lowest group occur near Anning Chou, (24° 56′ : 102° 29′) in the Yunnan Fu neighbourhood1 near Hungai, (25° 24′ 100° 24') at the head of the Mitu plain in Central Yunnan and north of the Yangtze on the road to Mankuan (26° 20′ 100° 24'), Yungpei T'ing district, where the conglomerates contain pebbles of jasper and red and green porphyrite3. Where the conglomerates are absent, the lowest subdivision is often distinguished by the vivid and rapidly changing colours of its weathered shaly bands; purple, reddish-purple and maroon, violet, red and pink, with layers of green, greenish-white and buff, through which thin ribs of speckled sandstone protrude. The prevailing tint of the soil is a bright Indian red1.

The middle subdivision, of far greater areal extent than either of the other two, is predominantly shaly, usually of reddish and reddish-purple shades, often stained, spotted and flecked with green and sometimes of a distinct marly character. True calcareous horizons do occur but they are exceedingly rare and invariably thin. Of secondary importance in bulk are the sandstone bands, generally thin, red and soft, though occasionally more massive and harder, when paler tints and speckled varieties are often seen, as well as thin quartzitic layers, interbedded with reddish and reddish-violet clay shales. Associated with the thicker sandstone bands are the salt deposits which make the Red Beds of such economic importance. At Houching, in the Tingyuan Hsien district, the mineral occurs in patches and strings in a hard, red sandstone band of which about 20 feet are mined, but at most of the other localities, scattered in various parts of the province, weak brines, from which the salt is obtained by evaporation to dryness, are drawn from wells sunk into the saliferous horizons. Detailed accounts of a number of these occurrences and of the processes of manufacture are given in my account of the mines and mineral resources of Yunnan. In Southern Yunnan there appears to be more variety in the sandstones of this middle subdivision than in the central part of the Province. Near Weiyuan Ting, for instance there occur soft, light red arkoses; hard, reddish-black sandstones with

1 Rec. Geol. Surv. Ind., XLIV, p. 114, (1914).

2 Rec. Geol. Surv. Ind., LIV, p. 74, (1923).

3 Rec. Geol. Surv. Ind., LIV, p. 325, (1923).

4 Rec. Geol. Surv. Ind., LIV, p. 309, (1923).

5 Mem. Geol. Surv. Ind., XLVII, pp. 156-180, (1923).

Rec. Geol. Surv. Ind., LIV, p. 311, (1923).

G

inclusions of dense red clay; hard, reddish marls showing dark, ochreous stains; banded light red and yellow sandstones; hard, white, quartzitic kinds and bluish-grey, fine-grained examples containing gritty bands of pea-sized quartz and felspar.

The third and uppermost division consists of rather soft, thick, red, fine-grained sandstones, passing sometimes into conglomerates, generally found on the crests of the ridges, always possessing quite low dips, in contrast with the generally high dipping and often contorted strata of the lower divisions, and in some cases lying practically horizontally disposed. This subdivision is only definitely known in and about the salt field of Central Yunnan and is particularly well exposed around Luféng Hsien (25° 8′ 102° 9′). Looking to the north-east from this city, a long escarpment crowned by these flat red sandstones stretches for a considerable distance, while in the open, tree-less valley to the south, good exposures exist. The red sandstones on the high ground between Hsiaoshihchiao (25° 14' 102° 8') and Heiching (25° 22′ 101° 47′) are perhaps of the same age, while the low dips of similar rocks some 50 miles further west, between Shachiao (25° 15′ : 101° 11′) and Tienshentang (25° 20′ 101° 3'), may, in the absence of contrary evidence, justify their inclusion in the same subdivision.

Such, in brief, are the main rock types of the Red Beds, the age of which will now be considered.

II.-AGE.

A. Opinions of Earlier observers.

Ludwig von Loczy traversed the Yunlung Basin in 1880 but made few comments on this portion of his journey, remarking, however, that its sandstones, clay shales Ludwig von Loczy, 1880. and occasional limestone conglomerates, with their very disturbed bedding and induced schistose cleavage, resembled the Mesozoic deposits of the provinces of Szechuan and Kwanghsi. On the section-traverse accompanying his notes the region is shown as occupied by Permo-Triassic rocks.1

1" Die wissenschaftlichen Ergebnisse der Reise des Grafen Bela Szechenyi in Ost Asien ", Vol. 1, pp. 762-770, Vienna, 1892,

P. A. Duclos, 1895-97, and M. A. Leclère, 189799.

tively attributed a

P. A. Duclos who travelled in China in 1895-97, directed attention to the red sandstones of the north-east of Yunnan and tentaPermian or Triassic age to them.1 M. A. Leclère, (1897-99) regarded the salt-bearing red rocks of Central Yunnan as Upper Permian and on his small map groups the Devonian, Carboniferous and Lower Permian together while indicating the Upper Permian and Trias under another colour.2

J. Deprat surveyed a large part of Eastern Yunnan in 1909-11, though the main group of Red Beds which he described are not continuous with those of Central Yunnan. J. Deprat, 1909-11. They consist of conglomerates overlain by sandstones and variegated marls, followed by basalts and andesites and are placed at the top of the Thuringian in the Upper Permian. They occur in four elongated, disconnected but overlapping bands which commencing to the south-east of Linan Fu (23° 37′ : 102° 50′), stretch for some 120 miles in a north-north-easterly direction towards the Kueichow border beyond Luliang (25° 3′: 103° 36'). The bands are narrow, usually attaining a maximum width of about five miles and tapering to the north and south, but the northern one rapidly broadens out beyond Lunan (24° 47′ 103° 17'), so that at its extreme known limits near Lulaing, though broken by a narrow zone of Devonian rocks, it is nearly 25 miles across.

Amongst many interesting sections described by Deprat the following may be mentioned; near Tsin-chouei-tang, 14 miles north of Ami Chou (23° 40′ 103° 16'), the conglomerates rest with high dips on the Middle Devonian and contain limestone pebbles with both Uralian and Permian fossils at Wou-lou-si-chou, 17 miles north-east of Linan Fu, the red sandstones are missing and the conglomerates, in this case resting on the Uralian and 210 metres (688 feet) thick, are followed by great thicknesses of basic rocks. and tuffs: near Loukhi, approximately 14 miles north-west of Milê Hsien (24° 24′ : 103° 26'), the red sandstones are over 300 metres (984 feet) and the underlying conglomerates 150 metres (492 feet), thick.

Deprat stresses the episodic and inconstant character of the individual sandstone bands, their frequent marly and gypseous

1 "La Mission Lyonnaise d'exploration commerciale en Chine ", pp. 283-314, Lyons, 1898. 2 Annales des Mines, Vol. XX, 9me Ser., pp. 287-492.