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 Siluiian graptolites in tAvo 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 tliat 

 the later Silurian rocks occuj^y an infolded pocket of the Ordo- 

 vician. In either case we are forced to look for the junction fur- 

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

 face of conglomerate C' is the junction. The 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 approximately 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 l)e noted. In the 

 Ordovician graptolite bed the dip is <S4 E, and the strike 

 exactly north and soutli, while in the Silurian graptolite 

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

 Between the^^e two l:)eds there is no stidden change of dip 

 or strike. 



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



of the strata east of C bxit 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 f-act 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 

 Cj^ as a basal conglomerate. 



