754 GEOLOGY OF THE NORTH-WEST COAST OF TASMANIA, 



the beach opposite the Northdown sluice may be seen at low 

 water mark an outcropping mass of quartzite, the only instance 

 of the occurrence of any of the ancient rocks between Badger 

 Head and the Clayton Rivulet, a distance of twenty-three miles. 

 The limestone of the River Don and Railton, with Orthoceratidce 

 and corals, and the Caroline Creek sandstones, with Dikelo- 

 cephalus and other trilobites, do not approach the coast-line. 

 The basaltic country already described extends across the Mersey 

 to within a short distance of the Mersey Bluff, and the low-lying 

 part of it is covered more or less thickly up to a height of about 

 twenty feet above high-water mark with wide-stretching beds of 

 shingle derived from the old altered sandstones, which no longer 

 crop out above sea-level. The diabase of which the Mersey 

 Bluff is the terminal point almost encircles the basalt of East 

 and West Devonport, crossing the river in a south-easterly 

 direction. On a saddle of this ridge, between Devonport and the 

 River Don, at an elevation of about 200 feet, are drift gravels 

 corresponding to those noted near Port Sorell. They are 

 older than the basalt, and presumably of Tertiary age. The 

 Mersey Bluff (Plate xxiv., fig. 1) is the most westerly exposure 

 along the coast of this noncrystalline igneous rock which, over 

 the whole of Eastern Tasmania, is never out of sight. Its struc- 

 ture is of the common rudely prismatic type, and a noticeable 

 feature is a regular system of joints showing on the tops of 

 exposed and weathered columns four feet or more in diameter, 

 and radiating from the centre to the circumference as if they 

 subdivided the mass into a series of thin plates. In one instance 

 where long weathering had affected the structure it was possible 

 to detach plates of the size of " Duchess " roofing slates, and 

 tapering from a feather edge to a thickness of less than a quarter 

 inch on the outside of the column. 



The bluff west of the Don is a mass of the ordinary olivine 

 basalt of the North-West Coast showing the typical columnar 

 structure. About a mile south of the bluff, patches of shale 

 and sandstone may be seen cropping out under the basalt, which 

 probably belong to the Mersey coal measures. Farther south 



