1918.] The Geotectonics of the Tertiary Irrawaddy Basin. 415 
the middle of the geosyncline, the strata near the margins 
were probably acquiring a slight dip eastward, through earth 
movements uplifting the ; 
The Irrawaddy Basin formed a continually sinking Geosyn- 
cline—The total thickness of the tertiary deposits of the Irra- 
waddy basin may be judged from the following estimates :— 
Paunggyi or Swelegyin Conglomerates, Lower Eocene. 
Thickness varies from 2,000 to 4,000 ft. 
Laungshe Shales, Lower to Middle Eocene. Thickness varies 
from 9,000 to 12,000 ft.. 
Tilin Sandstones, Middle Eocene. Thickness varies from 
2,000 to 5,000 ft. 
Tabyin Clay, Middle Eocene. Thickness about 5,000 ft. 
Pondaung Sandstone, Upper Eocene. Thickness varies from 
3,000 to 8,000 ft. 
Yaw Shales, Uppermost Eocene. Thickness varies from 
1,600 to 2, ft. : 
Shwezetaw Sandstones, Lower Oligocene. Thickness varies 
from 3,000 to 3,500 ft. 
Padaung Clays, Upper Oligocene. Thickness 1,500 ft. in 
Minbu ; represented by sandstones of fluviatile character 
in Pakokku. | : 
Pegu Series, Miocene. Thickness about 8,000 ft. in south 
Minbu; in Pakokku, this Series and the Oligocene to- 
gether are represented west of Pauk by fluviatile sand- 
stones about 7,000 ft. thick, 
Irrawaddy Series, Pontian to Pliocene. Thickness probably 
about 6,000 ft. 
From a study of the fossil contents in these beds, we must 
Suppose that in all probability, none of these deposits were laid 
_ down in water much deeper than 200 fathoms; it follows, of 
course, that there must have been continual subsidence in the 
Irrawaddy Basin, from the lower eocene right down to the close 
“°f the pliocene, in order to allow of these deposits being laid 
,_own. Had there been no subsidence, the sea must have silted 
Up and become dry land at a very early period. : 
It may be supposed that the ratio of rate of subsidence os 
tate of deposition was not an entirely pr reicggeoe a — 
, tesulting in such comparatively deepish water deposits as 
the Yaw Shales and Padaung Clay overlying shallow water de- 
Pectively. Sometimes the shallow sea has silted up, giving rise 
to red earth 
Mains of land vertebrata, etc. 
v ield mapping in north Minbu and Pakokku has gee 
‘Sot clearly that the red earth beds are discontinuous ew 
vmerous in the Pegu Series, and that they occur throug! 
