YENANGYOUNG. 77 



series were deposited in a most irregular manner, and that in the 

 same level a sudden change from sandy into argillaceous beds often 

 takes place within a distance of a few feet (see page 78). 



All that we can therefore do under these circumstances is to 

 investigate whether a certain sequence of beds can be recognized in 

 the boring registers which, with restrictions, may hold good for 

 the area of the Yenangyoung oil field ; but this will be discussed 

 together with the sections of the deep wells. 



Section 3.— Stratigraphy. 



A—General Features. 



(a) Unequality of sedimentation. 



Local unconformity and contemporaneous erosion. — One of the 

 most remarkable features of the oil field is the great inconstancy of 

 the strata. Only in very rare instances can a bed be followed for a 

 few miles, and even within that short distance it undergoes rapid 

 changes in its character, which renders it often extremely difficult 

 to be recognized. 



In the Aungban-yo or in the Ayatpo-yo good illustrations of 

 complete thinning out of beds may be observed. On the northern 

 slope of the former a bed of bluish somewhat sandy clay appears 

 quite suddenly within a series of sandstone ; it attains very quickly 

 the considerable thickness of 50 feet within a few feet, and. then, 

 rapidly thins out, disappearing towards east and north under 

 younger beds ; its final termination is not seen. The section 

 exhibited in the Aytapo-yo shows this feature still better. The 

 bed just mentioned is represented at its southern end by thinly 

 laminated bluish clay alternating with sandy strata, where its top 

 is about 50 feet above the bottom of the ravine. Proceeding north- 

 wards it dips slowly in that direction until it disappears, and is 

 replaced hy a series of soft, whitish or red sandstones in irregular thick 

 banks, which contain at least four distinct beds of bluish clay, trace- 



( 123 ) 



