A NOTE ON THE GEOLOGICAL RESULTS OF THE EXPEDITION

By A. M. HERON, D.Sc., F.G.S., Geological Survey of India.

The area geologically examined is somewhat over 8,000 square miles, comprising the Tibetan portion of the Arun drainage area, with, in the West, the headwaters of the Bhotia Kosi and its tributaries.

The circumstances of the Expedition were not favourable for work in any detail, but an endeavour was made to traverse and map as large an area as possible on a scale of ¼-inch to the mile, on skeleton maps very kindly furnished by Major Morshead and his surveyors as their plane-tabling proceeded; my work must therefore be considered as a geological reconnaissance pure and simple.

If I am accorded the privilege of accompanying the second Expedition, by which time Major Wheeler's map on a scale of 1-inch to the mile will be available, I hope to be able to make a detailed survey of the vicinity of Mount Everest and investigate the complicated inter-relationships of the metamorphosed sedimentaries and the associated gneisses and granites.

My survey continues to the Westward Sir Henry Hayden's work during the Tibet Expedition in 1903–4.

Geologically this area is divided into two broad divisions: (a) Tibetan and sedimentary, (b) Himalayan and crystalline, a distinction which is clearly displayed in the topography resulting from the underlying geological structure, for to the North we have the somewhat tame and lumpy mountains of Tibet contrasting with the higher, steeper and more rugged Himalayas on the South.

The Tibetan zone consists of an intensely folded succession of shales and limestones, with subordinate sandstone quartzites, the folds striking East-West and mainly lying over towards the South, showing that the movements which produced them came from the North.

The uppermost rocks consist of the Kampa system of Hayden, a great thickness of limestones, which, where the rocks have escaped alteration, yield an assemblage of fossils which determine their age as Cretaceous and Eocene.

Below these is a monotonous succession of shales, practically unfossiliferous, with occasional quartzites and limestones representing the Upper and Middle Jurassic with at the base beds probably belonging to the Lias.

These Jurassic shales are by far the most conspicuous formation in this part of Tibet, being repeated many times in complicated folds.

The Cretaceous-Eocene limestones form comparatively narrow bands, occurring as compressed synclines caught up in the folded complex of Jurassic shales.

Along the Southern border of the Tibetan zone, below the base of the Jurassic shales, is a great thickness (2,000 feet–3,000 feet) of thinly bedded limestones in which the fossils have been destroyed and the rocks themselves converted over considerable areas into crystalline limestones and calc-gneisses containing tremolite, epidote, tourmaline, etc., but still retaining their original bedded structure in the banding of the altered rock.

The absence of determinable fossils makes it impossible to determine the age of these with certainty, but from their lithological character and position in the sequence, it is possible that they correspond with the Tso Lhamo limestone in Sikkim (Lias) and the Kioto limestone of the Zangskar range (Lower Jurassic and Upper Trias).

The Himalayan and crystalline zone is essentially composed of foliated and banded biotite-gneiss, usually garnetiferous, on which lie, at comparatively low angles and with a general Northerly dip, the above-mentioned calc-gneisses.

These occur most abundantly to the North and West of Everest, in the Keprak, Rongbu, Hlalung and Rebu Valleys. The group of high peaks to the North-west of Everest (overlooking the Khombu Pass) is made up of these and intrusive schorl granite, and it would seem that the precipitous North-western face and spurs of Everest are the same.

The Eastern and North-eastern valleys, Chongphu, Kharta and Kama, which are in general at a lower level than the North-western valleys, are excavated in the biotite-gneiss. On the North-eastern face of Everest fresh snow was too abundant at the time of my visit to make out what the rocks were.

Associated with the limestones and calc-gneisses are quartzites and tourmaline-biotite schists which probably represent the lowest portions of the shales immediately overlying the limestones.

It is probable that the biotite-gneiss is an igneous rock intrusive in the calc-gneisses and schists, but this and many other puzzling features of the crystallines require more detailed study than I was able to give this year.

Both biotite-gneiss and metamorphosed sedimentaries are crowded with dykes and sills, of all dimensions, of schorl granite or pegmatite to such an extent that this granite is frequently the predominant rock. It is highly resistant to weathering and it is doubtless due to its presence in large amount that such comparatively soft rocks as the calc-gneisses take part in forming some of the highest summits.

In the same way the scattered peaks of over 20,000 feet on the watershed between the Arun and the Tsangpo owe their prominence to their being groups of veins of a very similar granite, differing in that it contains biotite in place of schorl. Around these separate centres of intrusion are areoles of metamorphism in which the Jurassic shales have been converted into slates and phyllites.

Economically the area traversed by the Expedition is devoid of interest. Barring a little copper staining on a few boulders on moraines no traces of ore were seen.