OUTLINES OF GEOLOGY OF YUNNAN. 59 



Eastern Yunnan in the Trias did not reach the northern Shan States 

 until Rhsetic and Jurassic times, when the Triassic deposits of the 

 eastern area were already undergoing uplift and denudation. 



The Jurassic system is unknown in Yunnan and the Cretaceous 

 has never been recognised anywhere in China proper. 



The Pliocene (Nan Tien Series). 



With the Upper Trias marine sedimentation ceased and the 

 only known deposits of a later age are the fluvio-lacustrine or lacus- 

 trine beds of late Tertiary times. Deposition is still in progress 

 in some of the lake basins so it is sometimes impossible to separate 

 the older deposits from the newer. The beds themselves consist 

 of sands, sand-rock, clays, pebble beds and conglomerates. Bands 

 of lignite are also known to occur in places. 



These continental conditions, unimportant from the stratigra- 

 phical point of view, witnessed epoch-making changes of another kind. 

 At a period which belongs to the early Himalayan phase, Yunnan 

 and the Shan States were involved in far-reaching folding movements 

 which have been worked out in detail by Deprat. Overthrusting 

 was common and the Yangtze region in the neighbourhood of the 

 great bend, was pushed forward on to the Yunnanese area further 

 south. A long period of peneplanation followed, the Himalayan 

 folds were planed away by denudation and eventually, probably 

 towards the end of Pliocene times, great faults cut across the region. 

 These caused most of the depressions, which during the subsequent 

 period of stability gave rise to the lakes wherein the late Tertiary 

 fresh-water deposits accumulated, while at the same time the river 

 valleys became choked with thick sandy deposits. The final phase, 

 a very decided uplift, is responsible for much of the present topo- 

 graphy. It also drained many of the lake basins and installed the 

 minor features of the existing hydrographic systems. 



e2 



