Art. VI. — The Geology of the Portland Pramontory, 

 Western Victoria. 



By G. S. Griffiths, F.G.S. 



[Read June 9, 1887.] 



The area wliicli I propose to describe is a promontory, 

 terminating in three bold rocky headlands, Capes Bridge- 

 water, Nelson, and Grant, and two open bays, and these 

 features jointly constitute one of the most southerly exten- 

 sions of Australia. The town of Portland, which gives its 

 name to this promontory, is situated on the eastern side of 

 its neck. 



If we take a map of the locality, and run a line from 

 Narrawong due west, until it cuts the beach at Discovery 

 Bay, it will mark the base of the promontory, which we 

 shall find to be about twenty-two miles across, while the 

 length of its coast line is some sixty miles. The promontory 

 is for the most part a low table land, which increases in 

 height as we go from north to south, and which has bold 

 bluffs for most of its sea margin. Where the coast is low, as 

 it is between Whaler's Bluff and Narrawong, the strand 

 crosses the site of a former shallow arm of the sea, the bed 

 of which has been elevated just sufiiciently to form dry 

 land, and the old margin of this ancient bay is formed in 

 part of bold bluffs, similar to those which edge so much 

 of the promontory. The surface of this tableland is very 

 undulating, which characteristic is, on the south-western 

 edge, largely due to the presence of sand dunes, and else- 

 where is the result of unequal erosion. Its highest points 

 are Mount Bichmond, 740 feet high, and the extinct Bridge- 

 water volcano, 449 feet. 



I.— Its External Relations. 



From a geological point of view, the Portland Promontory 

 is but a corner of a large area occupied by upraised seabeds 

 of Tertiary age. Some time in the Eocene, if not before 

 it, the south coast of Victoria and South Australia was 

 depressed, and the ocean extended several great arms for 

 considerable distances within the present shore line. One 



