156 J. T. Jatson: 



changing near Melbourne (? Morang) to a deep gorge, which has 

 all the characters of a young valley. The change, he remarks, 

 is due to the fact that the old valley has been filled up by a 

 flood of lava (i.e., the newer basalt), that the river has had 

 to cut its channel anew, and that, owing to the hardness of the 

 basalt, the river has corroded its bed along the junction between 

 the basalt and the softer silurian sandstones and shales. Prof. 

 Gregory also maintains^ that the King Parrot Creek and the 

 Plenty River were probably originally one consequent river. 

 This question is outside the scope of the present paper, and is 

 not therefore discussed. 



The remarks of both Prof. Gregory and Mr. Hart throw light 

 on the history of the Plenty. They do not furnish a complete 

 explanation of how the present conditions arose ; but it must 

 be remembered that they were only dealing in a general way 

 with the subject. Mr. Hart recognises possible changes in the 

 course of the upper Plenty ; while Prof. Gregory notes that 

 the old valley (along the present course) has had a stream of 

 lava poured into it, and that the river has had to re-excavate 

 between the basalt and the silurian. He also remarks on the 

 broad mature valley up stream, and the youthful valley lower 

 down ; but he does not suggest any change in the actual course 

 of the river. 



The Later History of the Plenty. 



An examination of the nature and relative position of the 

 various geological formations along the course of the Plenty is 

 necessary, in order to appreciate its later history. As already 

 mentioned, at Whittlesea and Yan Yean the river meanders 

 through an alluvial flood-plain of about a mile in width, until it 

 meets the newer basalt. This broad alluvial plain (together 

 with the widening of the upper tributaries) has no doubt been 

 caused in part by the hard basalt damming the stream ; or at 

 least so retarding vertical erosion that lateral erosion took 

 place, and built up the flood-plain. That the ujiper ])art of 

 the valley, however, was, prior to the flow of the ])asalt. con- 

 siderably developed, is indicated by the width of the old valley 



1 lb., i>. 113. 



