1918.| The Geotectonics of the Tertiary Irrawaddy Basin. 417 
tion of the geosyncline. These folds, it must always be re- 
membered, are not merely anticlines, but folds consisting of 
anticlines and synclines. Thus, if the Yenangyat-Singu Hills 
and the Gwegyo Hills are minor anticlinal folds, the narrow 
belt between them (about 7 to 8 miles broad) is a minor 
syncline. At the time that these folds were forming, there 
anticlines 
Such conditions might explain any local unconformities 
observed in the neighbourhood of the minor anticlinal folds of 
the Irrawaddy Basin. : 
in such cases we ought to be able to prove a dis- 
cordance of dip between the two unconformable series. 
And since no discordance of dip has been proved along 
the flanks of the anticlines of Yenangyaung and Yenangyat, 
I do not think that we can justifiably have recourse to this 
explanation in these particular cases. : 
The Sea Retreated to the South.—In every section which we 
*xamine, we find marine sediments at the base, earth beds and 
fluviatile sediments at the top. We may in fact state our 
observations as follows :-— 
(1) In every section there is change vertically upwards, 
from marine, through deltaic and fluviatile beds to 
red earth and red earthy conglomerates. 
(2) In every section from the Pondaung sandstone ™p- 
wards, there is a change horizontally along the 
from marine to fluviatile and red earth beds. 
Ancient Coast-Line.—If, as we suppose, the Yoma barrier 
"as rising while the Irrawaddy Basin was subsiding, there must 
ve been a belt along the flanks of the geosyncline, where there 
Was neither upward nordownward motion. — This line of rest was 
ancient coast-line, and as time went on, the coast-line ad- 
vanced to a south and east, as the subsiding area shifted 
lf then, in the process of field mapping, we travel along 
Pil grin parallel to the ancient coast-line, we would 
et to 
fone its strike. In such a case the r 
om useful and mappable horizons, as indeed they 
