164 J. T. J lit son: 



eastern side, which will account for its contraction. This 

 repeats a feature already noticed in the old valley at Morang, 

 due to the hard granite and indurated silurian. 



If the original Plenty followed the course suggested, its 

 western side was probably a low ridge connected with the 

 Morang Hills, and the ridge of silurian and tertiary strata at 

 Preston. Tliis assumed part of the ridge is covered by newer 

 basalt : and the question arises why such middle part should 

 be covered and the ends exposed. The southern end is bare, 

 probably on account of the basalt thinning out m its southward 

 flow. A point would be reached where the valleys alone could 

 receive the w^hole of the basaltic stream. As regards the Morang 

 Hills, reference has already been made to the possibility of their 

 upper portion forming a monadnock on the old peneplain. 

 These hills towards their centre (where the granite and indurated 

 silurian rocks occur) have a fairly even sky-line. To the north 

 and south, this line drops rather suddenly, and in the south it 

 quickly passes under the basalt. To the north, after the rapid 

 drop, the lower part runs northerly as an even ridge of about 

 the same height, or in the same line of slope, as the western 

 ridge of the Plenty between Whittlesea and Yan Yean, with 

 which it was at one time continuous. It is now broken by the 

 Barber's Creek Gap. The height of this ridge near South Yan 

 Yean (in section '2, Parish of Y^'an Y^'ean) is about 700 feet 

 above sea level. The top of the Morang Hills is probably at 

 least 100 feet higher. If the 700 feet ridge be continued in a 

 sloping line southward, it would meet at the southern end of 

 the hills, the lower ridge which soon passes beneath the basalt. 

 Thus if the upper part of the Morang Hills be treated as a 

 monadnock, the discrepancy between their height and that of 

 the assumed covered ridge to the south is somewhat explained. 

 In any event, however, the hard rocks would ultimately tend to 

 project above the surrounding softer ones. When it is remem- 

 bered that the newer basalt was high enough to pierce the ridge 

 forming the eastern boundary of the main basaltic mass, and 

 that this ridge is protected by caps of older basalt, it is not diffi- 

 c\dt to believe that a ridge lies buried between the Morang Hills 

 and Preston, especially if that ridge did not possess any hard 

 rocks as a protection against denudation. 



