RELATION OF STREAMS TO GEOLOGICAL STRUCTURE. 53 



draining in a different direction from that in which their 

 trunk stream is now flowing. 



While by no means denying the probability that Dr. 

 Taylor's theory correctly accounts for the phenomena in 

 certain cases, the author, from observations made in vari- 

 ous parts of New South Wales, lias been led to believe that 

 these boathook bends may at times originate otherwise 

 than by river capture, and may in fact represent features 

 of the normal development of a river system undisturbed 

 by accident of any kind. 



Given a homogeneous land surface with a seaward tilt, 

 the river systems will develop normally, the tributaries of 

 each stream having a symmetrical disposition towards it, 

 and joining it at angles which are obtuse down stream. In 

 such circumstances the presence of boathook bends would 

 almost certainly be conclusive evidence of river capture 

 and a breached divide. But where, as is very often the 

 case, the land surface is non-uniform in structure and in 

 the nature of its component geological units it is inevitable 

 that in the mature stages of the river's history there 

 should be considerable adjustment of streams to geological 

 structures, and this will particularly affect the tributaries 

 which come into existence after the steep gradients of the 

 original consequent stream have been worn down. There 

 will be a seeking out of softer strata, and an avoidance of 

 the more resistant geological units as far as possible. In 

 other words, the law of physiographic symmetry enunci- 

 ated by Taylor will not necessarily hold. This is well 

 exemplified in many of our New South Wales river systems, 

 as has been pointed nut by Mr. E. O. Andrews and others. 

 In particular the more or less meridional courses of the 

 streams heading in the Monaro region are surely related 

 to the geological structure of the country, and to the 

 disposition of hard and soft geological units. A glance at 



