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.