Page images

structure of the Red Beds here is that of an asymmetric syncline, whose axis runs east of Monghsaw. A comparatively small area of the Monghsaw valley, roughly that occupied by the paddy fields around the village, lies under beds of incoherent sand and clays which were, no doubt, deposited in a lake in late Tertiary times, probably in the Pliocene. Their interest lies in the fact that they were found to contain beds of peat and lignite, but these are likely to be of commercial value.


Lake deposits,


Phyllites and mica schists of the Nam Hka

The Red Beds of the Monghsaw valley extend for some distance to the west, beyond the crest of the high hill range on the west of the village, and their basal grits lie directly upon light green phyllites which Cover the valley. western slope of the range. The phyllites are soft and easily weathered into light red soil, contain frequent veins of quartz and dip to the east. They continue to the Nam Hka, whilst across that stream, on the west, they are seen to become coarser grained, in so far as they begin to show individual crystals of mica and approach mica-schist in general characteristics. For about two miles up the slope on this western side of the Nam Hka they retain an easterly dip, but beyond, the dip changes to west. Where the dip changes they contain a well developed bed of marble of pure white colour and fine grain. Further west and east of the crest of the high range, there occurs another strong bed of similar marble which was traced in a continuous line for about six miles. This bed shows a westerly dip and is separated from the former by mica-schists.

To the north both these marble bands appear to die out temporarily, but re-appear in a few isolated outcrops along the same strike, between Pangsuk (M 10)1 and Monghka (M 10).1 Monghka gneisses. Before reaching the latter village the micaschists disappear and give place to the fine-grained gneisses on which Monghka is built.

In Mr. Sondhi's opinion the Monghka gneiss owes its origin to the soaking of mica-schists by a granitic magma, which at the same time dissolved the greater portion of the limestone bands, and left only isolated bluffs of undissolved marble.

For about ten miles to the north of Monghka nothing but these gneisses occur, but among these, bluffs of marble were noticed in

1 Numbers in brackets are those of squares on the Sino-British Boundary Commission map.

several places, occurring approximately on the strike of the more regular beds to the south.

The greater part of the country between Panghsang and Lufang was found by Mr. Sondhi to be occupied by shales and phyllites of about the same age as those occurring in the eastern areas already described; they were invariably found to dip to the east, and near Hsonglong (J 15)1 and Vingnun (K 11)1 to contain interbedded lime

Panghsang-Luf ang


At Nahpan (H 11)1 there occurs a small basin of Red Beds and further north, near Vingmau (G 9)1 and Nya Wa (F 7)1 small patches of granite were mapped, intruded into phylGranite near Nya Wa. lites and shales. A study of the thermal effect of these intrusions on the sedimentary rocks proved interesting, in so far as it tended to show that the origin of mica-schists in the eastern areas was not to be attributed to the thermal effects of granitic intrusion alone. Near Nya Wa the effect of the intrusion appeared to be confined to a relatively narrow zone in which the sedimentary rocks were indurated and silicified.


North-eastern Circle.

98. During the field season 1936-37 the North-eastern Circle was composed of Dr. C. S. Fox, Superintending Geologist (in charge), Mr. A. M. N. Ghosh, Geologist, and Mr. V. R. R. R. Khedker, Assistant Geologist. The entire party was engaged on surveys in the Garo Hills and the Khasi and Jaintia Hills districts of Assam, which lie between the Brahmaputra valley to the north and the plains of Sylhet and Mymensingh to the south.


Garo Hills, Assam.

99. Dr. Fox, in December 1936, spent a few days days in Shillong assisting the Assam Government in an advisory capacity regard regard to questions of limestone and coal. He next spent a few days with Mr. A. M. N. Ghosh in the Pynursla (25° 18′: 91° 53′) and Laitlyngkot (25° 26′: 91° 50′) area of the Khasi Hills (Sheet 78 O) for the purpose of settling the problem of the age of certain conglomerates and investigating an occurrence of lithomarge which Mr. Ghosh had found. Eventually, Dr. Fox proceeded to his own field of work, in the middle of December, 1936, south of the Tura Range, Garo Hills (Sheet 78 K/S. E.).

1 Numbers in brackets are those of squares on the Sino-British Boundary Commission


As already reported in the General Report for 1936, [Rec. Geol. Surv. Ind., 72, p. 40, (1937)], Dr. Fox had indicated a large hidden. coalfield south of the Tura Range (Sheet 78 K) eastwards from the meridian of Tura (25° 30′ 90° 14′), in a narrow strip five miles wide to the Simsang river between Siju Songmong and Rewak Songmong, in which 500 million tons of coal should be recoverable from a single seam of good quality which appeared to underlie the area within one thousand feet of the surface. Dr. Fox has been able to confirm his opinion as to the geological structure of this area, so that should the coal seam not fluctuate greatly in thickness and quality the reserves estimated by him can be regarded as probable.

Before Dr. Fox returned to headquarters on the 20th February, 1937, Mr. V. R. R. R. Khedker worked under his instructions in the Simsang valley, from above Siju Songmong to below Baghmara, for two weeks.

100. During the field season Mr. Ghosh completed the mapping of sheet 78 0/15 and a portion of sheet 78 0/11. Most of the area is occupied by the rocks of the Shillong series and the remainder by epidiorite (Khasi greenstone), granite and dolerite. Outliers of

Khasi and Jaintia Hills district, Assam.

the Cretaceous and Eocene strata occur at many places.

In the eastern part of sheet 78 O/15, the Shillong series occupying the Umsi, the Umsong and the Umjar valleys consists of sericitequartzite, hornblende-, hornblende-biotite-, garnetiferous sericitechlorite-, and garnetiferous quartz-mica-schists, crumpled and closely folded together. Some of the hornblende-schists appear to be the metamorphic products of the Khasi greenstone. Quartzite with a slaty cleavage, quartzose schists and some slaty rocks occur on the plateau, east of a line joining Mawblang (25° 27′ 91° 57′) and Jong Kaksha (25° 28′ 91° 59′). Well bedded, jointed and variegated quartzite forms the principal member of the Shillong series in the other areas of the same sheet. As a rule the quartzite strikes N. N. E.-S. S. W. and has a high dip either towards the east or west, although in some places on the top of the plateau a very low dip is observed. On a jungle path, about a mile and a quarter W. N. W. of Wahkhen (25° 21' 91° 51'), the quartzite was found to carry hematite in a phyllitic matrix and loose pieces of a banded hematite-quartzite were noticed.

With the exception of conglomeratic quartzite on the Lum Marck (25° 25′: 91° 44′)—Nonglwai (25° 24′: 91° 44′) ridge, the Shillong

series in 78 0/11 is represented by argillaceous rocks consisting of shale, slate, carbonaceous phyllite and sericite-schist. The argillaceous series appears to underlie the quartzite, as it dips under the latter.

South and south-west of Mawphlang (25° 27': 91° 46′), the line of junction between the arenaceous and the argillaceous beds of the Shillong series is marked by a conglomerate, which is well developed in the Bagra valley, on the Laitsopliah (25° 23′ : 91° 46′) ridge and in the valley of the Umsohra. Its presence both on the hilltop and in the valley is explained by Mr. Ghosh as due to folding. The conglomerate is evidently absent west and north of Mawphlang. Huge blocks of conglomerate were noticed at several places in the eastern section of sheet 78 0/15.

The general structure of the area occupied by the Shillong rocks is believed by Mr. Ghosh to be a synclinorium, composed of asymmetric anticlines and synclines, which are well displayed amongst the quartzites; he notes, however, that folding is not noticeable in the beds of shale and slate. The strike of the system of folds lies between N. N. E.-S. S. W. to N. E.-S. W. and the rocks have a variable dip, mostly high and sometimes vertical. Local variations in the direction of strike occur, especially in the proximity of the granite masses.

The oldest igneous rock of the area is the Khasi greenstone, which is intimately associated with the rocks of the Shillong series. It has a dark grey to greenish colour and shows considerable variation in texture and composition. When fresh the greenstone is massive, well jointed and forms sheer cliffs. It exfoliates on weathering and decomposes into a characteristic orange-red clay. On the top of the plateau the greenstone forms low, elongated, dome-shaped hills.

The greenstone was evidently intruded into the Shillong series and was subsequently folded with them. Mr. Ghosh provisionally draws attention to the lithological similarity of the Shillong series and the associated epidiorite (greenstone) with the Iron Ore Series (Upper Dharwars) recognised by Dr. Dunn in North Singhbhum,


The next younger rock is the well-known Mylliem granite, of which the largest exposure, around Mylliem (25° 30′ 91° 49′), occupies an area of nearly 30 square miles. Mr. Ghosh has found inclusions of quartzite and schist and also xenoliths of greenstone in the granite, and in many places apophyses of the granite occur

in the foliation planes of the schists, while granite dykes are seen to cut the Khasi greenstone in the Umsohra valley. Basic segregations, i.e., ovoid and rounded patches, also occur in the granite. Locally the granite is found highly crushed and granulated, with a streaky texture, due to glide planes separating dark crushed minerals such as hornblende, chlorite and, in some cases, tourmaline, from lighter minerals. In Mr. Ghosh's opinion the metamorphism induced by the Mylliem granite intrusions on the Shillong rocks is very small.

The direction of the main vertical joints of the granite lies. between N. 5 E.-S. 5 W., to N. 60 E.-S. 60 W., thus showing a rough parallelism with the general strike of the Shillong series.

The youngest intrusive rocks are dolerite dykes that run both E. W and N. E.-S. W. and traverse all the above rocks. From the freshness of the augite in the dolerite it is thought by Mr. Ghosh to be the same as the Sylhet trap and he concludes that the dykes are of the same period of igneous activity. The age of the Sylhet trap is regarded as pre-Upper Cretaceous as it is overlaid discordantly by fossiliferous Upper Cretaceous rocks in places along the southern scarp of the Shillong plateau.

The Cretaceous strata of the Khasi Hills are more limited in distribution than was previously supposed. Mr Ghosh considers that the beds that were mapped as Cretaceous, in his area, north of latitude 25° 20′, belong to the Cherra stage of the Eocene. Early in December 1936, Dr. Fox, who was accompanied by Mr. Ghosh, studied some of the scarp sections of the younger sedimentaries on the plateau of Thang Jnat (25° 18′: 91° 54') and came to the conclusion that the true Cretaceous beds do not extend north of Khyrwet (25° 19′ 91° 54′), where the conglomerate, so well. developed in the stream beds and on the motor road E. S. E. and south-east of Pynursla (25° 19' 91° 54'), is represented by a few scattered pebbles (in the cliff section west of Khyrwet). Mr. Ghosh has found that on the Cherrapunji plateau the Cretaceous beds also rapidly thin out in a northerly direction and cannot be identified north of Laitryngew (25° 24′ : 91° 56′).

On the Thang Jnat plateau Mr. Ghosh has found that the Cherra sandstone, which succeeds the Cretaceous, ends near Ryngain (25° 22′ 91° 53'). Outliers of the Cherra sandstone, however, are seen at several places further north, as for example, that at

« PreviousContinue »