THE JURASSIC SYSTEM. 
31 
black slates, sandstones and conglomerates, dipping to the north : 
these have been more or less altered by the neighbouring granite, and 
are for the most part unfossiliferous. The only fossils met with 
occurred in a loose block of quartzite, found lying near the road a few 
hundred yards to the north of Gyamtsonang : this block must have 
been derived from one of the eastern spurs of Chomoyumoand consists 
of hard quartzite with crinoid stems and remains of brachiopods : the 
fossils, which are partly composed of soft limonitic material, are 
poorly preserved and are indeed merely casts. They include a species 
of Rhynchonella, resembling Rh. qiiadriplicata Zieten. 
From Gyamtsonang the road gradually ascends on to a mass of 
blocks of granite and quartzite, which have been brought down from 
Chomoyumo and its north-eastern spurs : thence a short ascent to the 
Kongra La brings into view the rolling hills of the Yarn plain bounded 
on the far north by black jagged ridges of slate and snow-capped 
granite peaks which form the watershed between the Yarn river and 
the northward-flowing tributaries of the Nyang Chu and the Tsangpo. 
From the Kongra La the road crosses a flat ridge for some two 
miles, then descends from the Sibu La to Giri. 
La^and Qiri '^""^''a ^^^^ ^j^^ prevailing rock is slate talus from the 
northern flanks of Chomoyumo, but near the Sibu 
La the hills are formed of old moraines of granite and quartzite, relics 
of a time when the drainage from the granite zone flowed northwards 
to the Yaru. Below the Sibu La, far to the east and west, are undulat- 
ing hills of shale, thrown into gentle folds, with here and there an 
isolated mass of limestone ; but most conspicuous of all is the 
southern scarp of the Kampa ridge, a synclinal of hard Cretaceous 
and Tertiary beds standing prominently out above the soft and easily 
weathered Jurassic shales. 
From the Lachen valley then the sequence is as follows : below 
are the conglomerates, quartzites and slates of the Chomoyumo spurs 
and the Kongra La: these dip to the north and eventually disappear 
under the moraines of the Sibu La. East of the descent from the 
Sibu La to Giri, a small group of hills is composed of dark shales, 
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