484 L. Dudley Stamj) — 



gradually to the shallow waters of the Gulf of Martaban. The 

 northern half of the Tertiary region is occupied by the valleys of the 

 Chindwin and the Irrawaddy on the west and east respectively. 

 Climatologically the Tertiary belt, being on the lee of the Arakan 

 Yoma and protected from the rain-bearing South- West Monsoon, 

 enjoys a dry and healthy climate. The rainfall is lowest — about 

 20 inches — in the latitude of Yenangyaung, giving rise to a tract of 

 semi-desert. The rainfall increases to the north and to the south, 

 as well as to the west as one approaches the Arakan Yoma, and* the 

 semi-desert with its thorny shrubs, Euphorbias, Cacti, and stiff- 

 looking Toddy Palms passes gradually into shrubland and then 

 into Monsoon Forest of teak-trees. Elevation and soil play an 

 important part in the ecology of the country. 



(3) The Fold Range of the Arakan Yoma forms an important 

 barrier bounding the Tertiary region on the west. It stretches 

 from the Naga Hills of Assam in the north to the extreme south- 

 western point of Burma in Cape Negrais, being continued by the 

 rugged islands of the Andaman and Nicobar groups into Sumatra 

 (Barissan Range) and Java. Topographically the range is still 

 largely unknown, large areas being unmapped in detail, whilst 

 geological knowledge depends on a few incomplete traverses. 

 Throughout the length of the range there is really no line of transverse 

 communication — not even a well-known forest track, and large areas 

 are uninhabited. As knowledge of their geology slowly accumulates 

 it becomes more and more certain that the structure of the Arakan 

 Yomas is that of a complex anticlinorium. The core consists of 

 pre-Tertiary rocks ^ (including at least Trias ^ and Cretaceous — 

 Cenomanian ^ and possibly Danian *). The core is flanked on 

 either side by Tertiary rocks. Certain facts of considerable 

 importance disengage themselves from the scattered scraps of avail- 

 able information : — 



(a) at or near the base of the Tertiary deposits on the eastern side 

 there is a very thick basal conglomerate (Paung-gyi Conglomerate) which 

 is Lower Eocene in age, 



(6) despite the fact that the Eocene deposits take part in the folding 

 and have been ridged into steep-limbed anticlines or overfolds (see Fig. 6), 

 there is always a marked change from the comparatively unaltered 

 Tertiary to the cleaved and puckered Cretaceous and earlier rocks, 



(c) there is a notable difference between the Tertiary deposits on the 

 two sides of the Arakan Yoma. 



One is forced to the conclusion that the great fold of the Arakan 

 Yoma was initiated in late Cretaceous times, and even as early as 

 the Eocene constituted a barrier between the areas of deposition on 



^ Axial Group, Mail Group, and part of the Negrais Group of Theobald, 

 Mem. G.8.I., vol. x, pt. ii, 1874. This paper of Theobald on the " Geology of 

 Pegu ", or Lower Burma, laid the real foundation of the study of the geology 

 of Burma, and is still largely followed. 



2 Mec. G.S.I., vol. xxxiv, 1906, p. 134. 



3 Mem. G.S.I. , vol. x, 1874, p. 123 (311). 



* Rec. G.S.I. , vol. xxxv, pt. ii, 1907, p. 119. 



