Physiography of Bulla Area. 331 



edition of the quarter sheet 7 S.E., places the junction one-eighth 

 of a mile to the west of this, and this is approximately the line 

 found independently by T. S. Hart (6). 



The presence of Silui-ian graptolites in two localities one and 

 a-half miles west of this junction, and in another one and a-half 

 miles N.N.W., strongly suggests that the junction shown in the 

 ^quarter sheets is incorrect. There is of course the possibility that 

 the later Silurian rocks occupy an infolded pocket of the Ordo- 

 vician. In either case we are forced to look for the ju^iction fur- 

 ther upstream. It seems to the writer probable that the western 

 face of conglomerate C is the junction. Tlie reasons for putting 

 it there are as follow : — 



(a) The junction is necessarily between dyke D and Column 



Gully, for at the former, Silurian graptolites and worms 

 are found, and at the latter, Ordovician graptolites. 

 These two places are approxinlately one and a-half miles 

 apart. No conclusive evidence is given by the dips and 

 strikes, for though slight variations occur, up-stream and 

 down-stream similar variations can be noted. In the 

 Ordovician graptolito "bed the dip is 84 E, and the strike 

 exactly north and south, while in the Silurian graptolite 

 beds the. dip is 77°E, and the strike 8° west of north. 

 Between these two beds there is no sudden change of dip 

 or strike. 



(b) The Silurian worm Trachyderma can be found in many 



of the strata east of C^, but not west of it, 

 {c) ''Ordovician worm impressions" are found in enormous 

 numbers, .from the Ordovician graptolite beds, where 

 they are associated with Diplograptus, right up to the 

 conglomerate C , but neither in it nor on the east side. 

 The fact that tliese marks suddenly cease at the con- 

 glomerate strongly suggests discontinuity of conditions. 

 .(d) The conglomerate itself is strictly conformable with • the 

 strata on the eastern side, but on the western the con- 

 tact is very irregular. The one drawback to the plac- 

 ing of the junction at ^ is the presence of a few pebbles 

 in the strata on the up-stream side of the conglomerate. 

 This suggests similarity of conditions, and though their 

 presence is not fatal, yet it makes one hesitate to accept 

 C^ as a basal conglomerate. 



