58 NOETLING : PETROLEUM IN BURMA. 



Besides the vertebrate fauna, silicified wood is extremely common 

 in the Irawadi series. Huge logs and smaller fragments may be 

 found almost anywhere throughout the beds. As I have expressed 

 my views with regard to the origin of the fossil wood in the paper 

 quoted, it is unnecessary to repeat them here. 



Excepting the two species Cyrena (Batissa) crawfurdi and 

 Cyrena (Batissa) petrolei which occur at the very base of the 

 Irawadi series, and which also descend lower down, the beds 

 forming the latter have so far been found to be absolutely destitute 

 of invertebrates. 



(c) Local subdivision.— In the neighbourhood of Yenangyoung, 

 the Irawadi series attains a thickness of 4,630 feet measured from 

 the base of the zone of Hippotherium antelopinum to the bank of 

 the Irawadi. It is, however, quite certain that this does not by any 

 means represent the total thickness of the division, which must be 

 much larger even near Yenangyoung, but, owing to the want of more 

 accurate maps, it was impossible to estimate the thickness with any 

 accuracy. 



It is beyond question that the subdivision of so lithologically 

 uniform a sequence of beds, of a known thickness of not less than 

 4,600 feet is extremely difficult, if not altogether impossible. It 

 seems, however, as if certain species of the vertebrate fauna were 

 limited to certain horizons. Whether I am right in this supposition 

 remains to be seen, when the Irawadi series has been studied 

 in greater detail in other parts of Burma ; but I am absolutely sure 

 that certain species are restricted in the neighbourhood of Yenan- 

 gyoung to the lower parts of the division, while others have so far 

 been only found in the upper parts. 



In descending order I distinguished three stages — 



1. Sandstone, apparently containing no vertebrate remains, but 



plenty of fossil Wood. 



2. Zone of Mastodon latidens and Hippopotamus irravadicus. 



3. Zone of Hippotherium antelopinum and Acerotherium perimense. 



The three horizons are of very unequal value as regards thick- 

 ness, inasmuch as the lower one is of not more than 20 to 25 feet in 

 ( 104 ) 



