BY L. KEITH WARD, B.A., B.E. j.^ 



earl}^ Palaezoic time has exerted a powerful influence 

 in the moulding of the land forms of to-day. 



The overlying sediments have been worn down, and 

 great gorges have been carved through the diabase down 

 on to the Pre-Cambrian bedrock. 



On the flanks of these valleys there remain in some 

 places the remnants of former sedimentary basins. For 

 instance, on the north-western face of Mount Arrow- 

 smith lies the fragment of a Silurian sandstone formation! 

 dipping to the westward at a low angle. The same for- 

 mation has been to a greater degree preserved in the 

 area lying to the northward of the Raglan Range. And 

 even here the Nelson River is now steadily removing 

 the softer sandstone, while the relatively harder quartz 

 schist of the Raglan Range is forcing the river channel 

 ever to the northward. 



Still more significant of the surviving control exerted 

 by the lower Palaeozoic physiography over prcbCfit land 

 forms is the distribution of the limestone in the valleys 

 of the western rivers — especially of the Gordon River 

 and its tributaries. Recent exploratory work has proved 

 a remarkable restriction of limestone (all the exposures 

 of which appear to be of Ordovician age) to the bottom 

 of some of these valle3'-s. In some cases the limestone 

 is only visible actually in the beds of the rivers which 

 traverse these valleys. These latter remarks apply to 

 the Jane and Denison River valleys. 



In the case of the Surprise River, which occupies 

 the gorge between Mount King William ist and the 

 Loddon Range, the limestone has been cut through by 

 the corrosive action of the river, and is now situated a. 

 few feet above river level. 



The manner in which these ancient sediments con- 

 form to the present physiographical outlines is at least 

 suggestive of the theory here advanced. 



But it must be remembered that an explanation of 

 the phenomena exhibited by such a restricted area can- 

 not be applied beyond the limits of this area. The 

 Ordovician Hmestone of Western Tasmania is not 

 always found in the depths of the valleys, and occur- 

 rences which might seem to contradict the hypothesis 

 here put forward are probably to be easily explained in 

 different ways. The corrosive action of the different 

 rivers may have outstripped erosion and left the lime- 



