ics 
1923. ] Ecology of Riverine Tract of Burma, 93 
extreme temperatures recorded in January were 489° and 
897°, the corresponding figures in April being 70:9 and 
109°5°. 
Variation in the amount of direct sunshine plays a pe 
unimportant part in controlling the minor changes in 
vegetation. The hill-ranges run roughly from NNW. to SSB. 
and, being formed of steeply-dipping rocks, their slopes often 
reach 30° or more. Consequently the western flanks are 
illuminated by direct sunshine for several hours less per 
day and the vegetation on these slopes is often distinctly 
richer and more hygrophilous. Sunlight often only penetrates 
into the eae chaungs in these hills for a few hours per day. 
cold season low lying tracts of clay are often 
subjected - thick morning mists, which may not clear away 
until 930 am. The dew-fall in the same season is very 
heavy and these two factors seem to control to a large ex- 
tent the period of leaf-fall. 
‘or the purposes of this study the geological formations 
present may be tabulated as follows :— 
‘DRIFT’ FORMATIONS (POST-TERTIARY). 
Alluvium of the River Irrawaddy. 
— Chaung} oe and Hill-wash, 
Binkad Gra 
‘SOLID’ FORMATIONS (TERTIARY). 
Irrawadian System (Mio-Pliocene)—mainly coarse, in- 
coherent sands, usually slightly ferruginous, with 
binge of coarse, ferruginous cong one ee ae 
8 grey 
or mottled red and grey plastic clays. 
Pegu System (Oligo-Miocene) :— 
Upper Group—alternating beds of rather fine-grain- 
ed sands, locally hardened into resistant san si 
stones, and sandy or shaley clays. This gro 
includes the exposed Pegu rocks of the Oilfields 
of Yenangyaung, and Minbu; and the Prome 
Beds, ae Clay, Pyalo and Ayaukistng Beds 
of Lowe a. 
Middle ie ane a thick mass of indistinctly bed- 
ubbly shales or stiff cla ay, non-sandy. 
g 
and shales tema the Upper Group, except 

! Chaung (Burmese), stream or stream course. 
